hibernate-orm/reference/en/modules/basic_mapping.xml

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<chapter id="mapping">
<title>Basic O/R Mapping</title>
<sect1 id="mapping-declaration" revision="1">
<title>Mapping declaration</title>
<para>
Object/relational mappings are usually defined in an XML document. The mapping
document is designed to be readable and hand-editable. The mapping language is
Java-centric, meaning that mappings are constructed around persistent class
declarations, not table declarations.
</para>
<para>
Note that, even though many Hibernate users choose to write the XML by hand,
a number of tools exist to generate the mapping document, including XDoclet,
Middlegen and AndroMDA.
</para>
<para>
Lets kick off with an example mapping:
</para>
<programlisting id="mapping-declaration-ex1" revision="1"><![CDATA[<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class name="Cat"
table="cats"
discriminator-value="C">
<id name="id">
<generator class="native"/>
</id>
<discriminator column="subclass"
type="character"/>
<property name="weight"/>
<property name="birthdate"
type="date"
not-null="true"
update="false"/>
<property name="color"
type="eg.types.ColorUserType"
not-null="true"
update="false"/>
<property name="sex"
not-null="true"
update="false"/>
<property name="litterId"
column="litterId"
update="false"/>
<many-to-one name="mother"
column="mother_id"
update="false"/>
<set name="kittens"
inverse="true"
order-by="litter_id">
<key column="mother_id"/>
<one-to-many class="Cat"/>
</set>
<subclass name="DomesticCat"
discriminator-value="D">
<property name="name"
type="string"/>
</subclass>
</class>
<class name="Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>]]></programlisting>
<para>
We will now discuss the content of the mapping document. We will only describe the
document elements and attributes that are used by Hibernate at runtime. The mapping
document also contains some extra optional attributes and elements that affect the
database schemas exported by the schema export tool. (For example the <literal>
not-null</literal> attribute.)
</para>
<sect2 id="mapping-declaration-doctype" revision="2">
<title>Doctype</title>
<para>
All XML mappings should declare the doctype shown. The actual DTD may be found
at the URL above, in the directory <literal>hibernate-x.x.x/src/org/hibernate
</literal> or in <literal>hibernate3.jar</literal>. Hibernate will always look for
the DTD in its classpath first. If you experience lookups of the DTD using an
Internet connection, check your DTD declaration against the contents of your
claspath.
</para>
</sect2>
<sect2 id="mapping-declaration-mapping" revision="3">
<title>hibernate-mapping</title>
<para>
This element has several optional attributes. The <literal>schema</literal> and
<literal>catalog</literal> attributes specify that tables referred to in this mapping
belong to the named schema and/or catalog. If specified, tablenames will be qualified
by the given schema and catalog names. If missing, tablenames will be unqualified.
The <literal>default-cascade</literal> attribute specifies what cascade style
should be assumed for properties and collections which do not specify a
<literal>cascade</literal> attribute. The <literal>auto-import</literal> attribute lets us
use unqualified class names in the query language, by default.
</para>
<programlistingco>
<areaspec>
<area id="hm1" coords="2 55"/>
<area id="hm2" coords="3 55"/>
<area id="hm3" coords="4 55"/>
<area id="hm4" coords="5 55"/>
<area id="hm5" coords="6 55"/>
<area id="hm6" coords="7 55"/>
<area id="hm7" coords="8 55"/>
</areaspec>
<programlisting><![CDATA[<hibernate-mapping
schema="schemaName"
catalog="catalogName"
default-cascade="cascade_style"
default-access="field|property|ClassName"
default-lazy="true|false"
auto-import="true|false"
package="package.name"
/>]]></programlisting>
<calloutlist>
<callout arearefs="hm1">
<para>
<literal>schema</literal> (optional): The name of a database schema.
</para>
</callout>
<callout arearefs="hm2">
<para>
<literal>catalog</literal> (optional): The name of a database catalog.
</para>
</callout>
<callout arearefs="hm3">
<para>
<literal>default-cascade</literal> (optional - defaults to <literal>none</literal>):
A default cascade style.
</para>
</callout>
<callout arearefs="hm4">
<para>
<literal>default-access</literal> (optional - defaults to <literal>property</literal>):
The strategy Hibernate should use for accessing all properties. Can be a custom
implementation of <literal>PropertyAccessor</literal>.
</para>
</callout>
<callout arearefs="hm5">
<para>
<literal>default-lazy</literal> (optional - defaults to <literal>true</literal>):
The default value for unspecifed <literal>lazy</literal> attributes of class and
collection mappings.
</para>
</callout>
<callout arearefs="hm6">
<para>
<literal>auto-import</literal> (optional - defaults to <literal>true</literal>):
Specifies whether we can use unqualified class names (of classes in this mapping)
in the query language.
</para>
</callout>
<callout arearefs="hm7">
<para>
<literal>package</literal> (optional): Specifies a package prefix to assume for
unqualified class names in the mapping document.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
If you have two persistent classes with the same (unqualified) name, you should set
<literal>auto-import="false"</literal>. Hibernate will throw an exception if you attempt
to assign two classes to the same "imported" name.
</para>
<para>
Note that the <literal>hibernate-mapping</literal> element allows you to nest
several persistent <literal>&lt;class&gt;</literal> mappings, as shown above.
It is however good practice (and expected by some tools) to map only a single
persistent class (or a single class hierarchy) in one mapping file and name
it after the persistent superclass, e.g. <literal>Cat.hbm.xml</literal>,
<literal>Dog.hbm.xml</literal>, or if using inheritance,
<literal>Animal.hbm.xml</literal>.
</para>
</sect2>
<sect2 id="mapping-declaration-class" revision="3">
<title>class</title>
<para>
You may declare a persistent class using the <literal>class</literal> element:
</para>
<programlistingco>
<areaspec>
<area id="class1" coords="2 55"/>
<area id="class2" coords="3 55" />
<area id="class3" coords="4 55"/>
<area id="class4" coords="5 55" />
<area id="class5" coords="6 55"/>
<area id="class6" coords="7 55" />
<area id="class7" coords="8 55"/>
<area id="class8" coords="9 55" />
<area id="class9" coords="10 55" />
<area id="class10" coords="11 55"/>
<area id="class11" coords="12 55"/>
<area id="class12" coords="13 55"/>
<area id="class13" coords="14 55"/>
<area id="class14" coords="15 55"/>
<area id="class15" coords="16 55"/>
<area id="class16" coords="17 55"/>
<area id="class17" coords="18 55"/>
<area id="class18" coords="19 55"/>
<area id="class19" coords="20 55"/>
<area id="class20" coords="21 55"/>
<area id="class21" coords="22 55"/>
</areaspec>
<programlisting><![CDATA[<class
name="ClassName"
table="tableName"
discriminator-value="discriminator_value"
mutable="true|false"
schema="owner"
catalog="catalog"
proxy="ProxyInterface"
dynamic-update="true|false"
dynamic-insert="true|false"
select-before-update="true|false"
polymorphism="implicit|explicit"
where="arbitrary sql where condition"
persister="PersisterClass"
batch-size="N"
optimistic-lock="none|version|dirty|all"
lazy="true|false"
entity-name="EntityName"
check="arbitrary sql check condition"
rowid="rowid"
subselect="SQL expression"
abstract="true|false"
node="element-name"
/>]]></programlisting>
<calloutlist>
<callout arearefs="class1">
<para>
<literal>name</literal> (optional): The fully qualified Java class name of the
persistent class (or interface). If this attribute is missing, it is assumed
that the mapping is for a non-POJO entity.
</para>
</callout>
<callout arearefs="class2">
<para>
<literal>table</literal> (optional - defaults to the unqualified class name): The
name of its database table.
</para>
</callout>
<callout arearefs="class3">
<para>
<literal>discriminator-value</literal> (optional - defaults to the class name): A value
that distiguishes individual subclasses, used for polymorphic behaviour. Acceptable
values include <literal>null</literal> and <literal>not null</literal>.
</para>
</callout>
<callout arearefs="class4">
<para>
<literal>mutable</literal> (optional, defaults to <literal>true</literal>): Specifies
that instances of the class are (not) mutable.
</para>
</callout>
<callout arearefs="class5">
<para>
<literal>schema</literal> (optional): Override the schema name specified by
the root <literal>&lt;hibernate-mapping&gt;</literal> element.
</para>
</callout>
<callout arearefs="class6">
<para>
<literal>catalog</literal> (optional): Override the catalog name specified by
the root <literal>&lt;hibernate-mapping&gt;</literal> element.
</para>
</callout>
<callout arearefs="class7">
<para>
<literal>proxy</literal> (optional): Specifies an interface to use for lazy
initializing proxies. You may specify the name of the class itself.
</para>
</callout>
<callout arearefs="class8">
<para>
<literal>dynamic-update</literal> (optional, defaults to <literal>false</literal>):
Specifies that <literal>UPDATE</literal> SQL should be generated at runtime and
contain only those columns whose values have changed.
</para>
</callout>
<callout arearefs="class9">
<para>
<literal>dynamic-insert</literal> (optional, defaults to <literal>false</literal>):
Specifies that <literal>INSERT</literal> SQL should be generated at runtime and
contain only the columns whose values are not null.
</para>
</callout>
<callout arearefs="class10">
<para>
<literal>select-before-update</literal> (optional, defaults to <literal>false</literal>):
Specifies that Hibernate should <emphasis>never</emphasis> perform an SQL <literal>UPDATE</literal>
unless it is certain that an object is actually modified. In certain cases (actually, only
when a transient object has been associated with a new session using <literal>update()</literal>),
this means that Hibernate will perform an extra SQL <literal>SELECT</literal> to determine
if an <literal>UPDATE</literal> is actually required.
</para>
</callout>
<callout arearefs="class11">
<para>
<literal>polymorphism</literal> (optional, defaults to <literal>implicit</literal>):
Determines whether implicit or explicit query polymorphism is used.
</para>
</callout>
<callout arearefs="class12">
<para>
<literal>where</literal> (optional) specify an arbitrary SQL <literal>WHERE</literal>
condition to be used when retrieving objects of this class
</para>
</callout>
<callout arearefs="class13">
<para>
<literal>persister</literal> (optional): Specifies a custom <literal>ClassPersister</literal>.
</para>
</callout>
<callout arearefs="class14">
<para>
<literal>batch-size</literal> (optional, defaults to <literal>1</literal>) specify a "batch size"
for fetching instances of this class by identifier.
</para>
</callout>
<callout arearefs="class15">
<para>
<literal>optimistic-lock</literal> (optional, defaults to <literal>version</literal>):
Determines the optimistic locking strategy.
</para>
</callout>
<callout arearefs="class16">
<para>
<literal>lazy</literal> (optional): Lazy fetching may be completely disabled by setting
<literal>lazy="false"</literal>.
</para>
</callout>
<callout arearefs="class17">
<para>
<literal>entity-name</literal> (optional, defaults to the class name): Hibernate3
allows a class to be mapped multiple times (to different tables, potentially),
and allows entity mappings that are represented by Maps or XML at the Java level.
In these cases, you should provide an explicit arbitrary name for the entity. See
<xref linkend="persistent-classes-dynamicmodels"/> and <xref linkend="xml"/>
for more information.
</para>
</callout>
<callout arearefs="class18">
<para>
<literal>check</literal> (optional): A SQL expression used to generate a multi-row
<emphasis>check</emphasis> constraint for automatic schema generation.
</para>
</callout>
<callout arearefs="class19">
<para>
<literal>rowid</literal> (optional): Hibernate can use so called ROWIDs on databases
which support. E.g. on Oracle, Hibernate can use the <literal>rowid</literal> extra
column for fast updates if you set this option to <literal>rowid</literal>. A ROWID
is an implementation detail and represents the physical location of a stored tuple.
</para>
</callout>
<callout arearefs="class20">
<para>
<literal>subselect</literal> (optional): Maps an immutable and read-only entity
to a database subselect. Useful if you want to have a view instead of a base table,
but don't. See below for more information.
</para>
</callout>
<callout arearefs="class21">
<para>
<literal>abstract</literal> (optional): Used to mark abstract superclasses in
<literal>&lt;union-subclass&gt;</literal> hierarchies.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
It is perfectly acceptable for the named persistent class to be an interface. You would then
declare implementing classes of that interface using the <literal>&lt;subclass&gt;</literal>
element. You may persist any <emphasis>static</emphasis> inner class. You should specify the
class name using the standard form ie. <literal>eg.Foo$Bar</literal>.
</para>
<para>
Immutable classes, <literal>mutable="false"</literal>, may not be updated or deleted by the
application. This allows Hibernate to make some minor performance optimizations.
</para>
<para>
The optional <literal>proxy</literal> attribute enables lazy initialization of persistent
instances of the class. Hibernate will initially return CGLIB proxies which implement
the named interface. The actual persistent object will be loaded when a method of the
proxy is invoked. See "Proxies for Lazy Initialization" below.
</para>
<para><emphasis>Implicit</emphasis> polymorphism means that instances of the class will be returned
by a query that names any superclass or implemented interface or the class and that instances
of any subclass of the class will be returned by a query that names the class itself.
<emphasis>Explicit</emphasis> polymorphism means that class instances will be returned only
by queries that explicitly name that class and that queries that name the class will return
only instances of subclasses mapped inside this <literal>&lt;class&gt;</literal> declaration
as a <literal>&lt;subclass&gt;</literal> or <literal>&lt;joined-subclass&gt;</literal>. For
most purposes the default, <literal>polymorphism="implicit"</literal>, is appropriate.
Explicit polymorphism is useful when two different classes are mapped to the same table
(this allows a "lightweight" class that contains a subset of the table columns).
</para>
<para>
The <literal>persister</literal> attribute lets you customize the persistence strategy used for
the class. You may, for example, specify your own subclass of
<literal>org.hibernate.persister.EntityPersister</literal> or you might even provide a
completely new implementation of the interface
<literal>org.hibernate.persister.ClassPersister</literal> that implements persistence via,
for example, stored procedure calls, serialization to flat files or LDAP. See
<literal>org.hibernate.test.CustomPersister</literal> for a simple example (of "persistence"
to a <literal>Hashtable</literal>).
</para>
<para>
Note that the <literal>dynamic-update</literal> and <literal>dynamic-insert</literal>
settings are not inherited by subclasses and so may also be specified on the
<literal>&lt;subclass&gt;</literal> or <literal>&lt;joined-subclass&gt;</literal> elements.
These settings may increase performance in some cases, but might actually decrease
performance in others. Use judiciously.
</para>
<para>
Use of <literal>select-before-update</literal> will usually decrease performance. It is very
useful to prevent a database update trigger being called unnecessarily if you reattach a
graph of detached instances to a <literal>Session</literal>.
</para>
<para>
If you enable <literal>dynamic-update</literal>, you will have a choice of optimistic
locking strategies:
</para>
<itemizedlist>
<listitem>
<para>
<literal>version</literal> check the version/timestamp columns
</para>
</listitem>
<listitem>
<para>
<literal>all</literal> check all columns
</para>
</listitem>
<listitem>
<para>
<literal>dirty</literal> check the changed columns, allowing some concurrent updates
</para>
</listitem>
<listitem>
<para>
<literal>none</literal> do not use optimistic locking
</para>
</listitem>
</itemizedlist>
<para>
We <emphasis>very</emphasis> strongly recommend that you use version/timestamp
columns for optimistic locking with Hibernate. This is the optimal strategy with
respect to performance and is the only strategy that correctly handles modifications
made to detached instances (ie. when <literal>Session.merge()</literal> is used).
</para>
<para>
There is no difference between a view and a base table for a Hibernate mapping, as
expected this is transparent at the database level (note that some DBMS don't support
views properly, especially with updates). Sometimes you want to use a view, but can't
create one in the database (ie. with a legacy schema). In this case, you can map an
immutable and read-only entity to a given SQL subselect expression:
</para>
<programlisting><![CDATA[<class name="Summary">
<subselect>
select item.name, max(bid.amount), count(*)
from item
join bid on bid.item_id = item.id
group by item.name
</subselect>
<synchronize table="item"/>
<synchronize table="bid"/>
<id name="name"/>
...
</class>]]></programlisting>
<para>
Declare the tables to synchronize this entity with, ensuring that auto-flush happens
correctly, and that queries against the derived entity do not return stale data.
The <literal>&lt;subselect&gt;</literal> is available as both as an attribute and
a nested mapping element.
</para>
</sect2>
<sect2 id="mapping-declaration-id" revision="4">
<title>id</title>
<para>
Mapped classes <emphasis>must</emphasis> declare the primary key column of the database
table. Most classes will also have a JavaBeans-style property holding the unique identifier
of an instance. The <literal>&lt;id&gt;</literal> element defines the mapping from that
property to the primary key column.
</para>
<programlistingco>
<areaspec>
<area id="id1" coords="2 70"/>
<area id="id2" coords="3 70" />
<area id="id3" coords="4 70"/>
<area id="id4" coords="5 70" />
<area id="id5" coords="6 70" />
</areaspec>
<programlisting><![CDATA[<id
name="propertyName"
type="typename"
column="column_name"
unsaved-value="null|any|none|undefined|id_value"
access="field|property|ClassName">
node="element-name|@attribute-name|element/@attribute|."
<generator class="generatorClass"/>
</id>]]></programlisting>
<calloutlist>
<callout arearefs="id1">
<para>
<literal>name</literal> (optional): The name of the identifier property.
</para>
</callout>
<callout arearefs="id2">
<para>
<literal>type</literal> (optional): A name that indicates the Hibernate type.
</para>
</callout>
<callout arearefs="id3">
<para>
<literal>column</literal> (optional - defaults to the property name): The
name of the primary key column.
</para>
</callout>
<callout arearefs="id4">
<para>
<literal>unsaved-value</literal> (optional - defaults to a "sensible" value):
An identifier property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from detached instances that were saved or loaded
in a previous session.
</para>
</callout>
<callout arearefs="id5">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy Hibernate should use for accessing the property value.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
If the <literal>name</literal> attribute is missing, it is assumed that the class has no
identifier property.
</para>
<para>
The <literal>unsaved-value</literal> attribute is almost never needed in Hibernate3.
</para>
<para>
There is an alternative <literal>&lt;composite-id&gt;</literal> declaration to allow access to
legacy data with composite keys. We strongly discourage its use for anything else.
</para>
<sect3 id="mapping-declaration-id-generator" revision="2">
<title>Generator</title>
<para>
The optional <literal>&lt;generator&gt;</literal> child element names a Java class used
to generate unique identifiers for instances of the persistent class. If any parameters
are required to configure or initialize the generator instance, they are passed using the
<literal>&lt;param&gt;</literal> element.
</para>
<programlisting><![CDATA[<id name="id" type="long" column="cat_id">
<generator class="org.hibernate.id.TableHiLoGenerator">
<param name="table">uid_table</param>
<param name="column">next_hi_value_column</param>
</generator>
</id>]]></programlisting>
<para>
All generators implement the interface <literal>org.hibernate.id.IdentifierGenerator</literal>.
This is a very simple interface; some applications may choose to provide their own specialized
implementations. However, Hibernate provides a range of built-in implementations. There are shortcut
names for the built-in generators:
<variablelist>
<varlistentry>
<term><literal>increment</literal></term>
<listitem>
<para>
generates identifiers of type <literal>long</literal>, <literal>short</literal> or
<literal>int</literal> that are unique only when no other process is inserting data
into the same table.
<emphasis>Do not use in a cluster.</emphasis>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>identity</literal></term>
<listitem>
<para>
supports identity columns in DB2, MySQL, MS SQL Server, Sybase and
HypersonicSQL. The returned identifier is of type <literal>long</literal>,
<literal>short</literal> or <literal>int</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>sequence</literal></term>
<listitem>
<para>
uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator
in Interbase. The returned identifier is of type <literal>long</literal>,
<literal>short</literal> or <literal>int</literal>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>hilo</literal></term>
<listitem>
<para id="mapping-declaration-id-hilodescription" revision="1">
uses a hi/lo algorithm to efficiently generate identifiers of
type <literal>long</literal>, <literal>short</literal> or <literal>int</literal>,
given a table and column (by default <literal>hibernate_unique_key</literal> and
<literal>next_hi</literal> respectively) as a source of hi values. The hi/lo
algorithm generates identifiers that are unique only for a particular database.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>seqhilo</literal></term>
<listitem>
<para>
uses a hi/lo algorithm to efficiently generate identifiers of type
<literal>long</literal>, <literal>short</literal> or <literal>int</literal>,
given a named database sequence.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>uuid</literal></term>
<listitem>
<para>
uses a 128-bit UUID algorithm to generate identifiers of type string,
unique within a network (the IP address is used). The UUID is encoded
as a string of hexadecimal digits of length 32.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>guid</literal></term>
<listitem>
<para>
uses a database-generated GUID string on MS SQL Server and MySQL.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>native</literal></term>
<listitem>
<para>
picks <literal>identity</literal>, <literal>sequence</literal> or
<literal>hilo</literal> depending upon the capabilities of the
underlying database.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>assigned</literal></term>
<listitem>
<para>
lets the application to assign an identifier to the object before
<literal>save()</literal> is called. This is the default strategy
if no <literal>&lt;generator&gt;</literal> element is specified.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>select</literal></term>
<listitem>
<para>
retrieves a primary key assigned by a database trigger by selecting
the row by some unique key and retrieving the primary key value.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>foreign</literal></term>
<listitem>
<para>
uses the identifier of another associated object. Usually used in conjunction
with a <literal>&lt;one-to-one&gt;</literal> primary key association.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect3>
<sect3 id="mapping-declaration-id-hilo" revision="1">
<title>Hi/lo algorithm</title>
<para>
The <literal>hilo</literal> and <literal>seqhilo</literal> generators provide two alternate
implementations of the hi/lo algorithm, a favorite approach to identifier generation. The
first implementation requires a "special" database table to hold the next available "hi" value.
The second uses an Oracle-style sequence (where supported).
</para>
<programlisting><![CDATA[<id name="id" type="long" column="cat_id">
<generator class="hilo">
<param name="table">hi_value</param>
<param name="column">next_value</param>
<param name="max_lo">100</param>
</generator>
</id>]]></programlisting>
<programlisting><![CDATA[<id name="id" type="long" column="cat_id">
<generator class="seqhilo">
<param name="sequence">hi_value</param>
<param name="max_lo">100</param>
</generator>
</id>]]></programlisting>
<para>
Unfortunately, you can't use <literal>hilo</literal> when supplying your own
<literal>Connection</literal> to Hibernate. When Hibernate is using an application
server datasource to obtain connections enlisted with JTA, you must properly configure
the <literal>hibernate.transaction.manager_lookup_class</literal>.
</para>
</sect3>
<sect3 id="mapping-declaration-id-uuid">
<title>UUID algorithm</title>
<para>
The UUID contains: IP address, startup time of the JVM (accurate to a quarter
second), system time and a counter value (unique within the JVM). It's not
possible to obtain a MAC address or memory address from Java code, so this is
the best we can do without using JNI.
</para>
</sect3>
<sect3 id="mapping-declaration-id-sequences">
<title>Identity columns and sequences</title>
<para>
For databases which support identity columns (DB2, MySQL, Sybase, MS SQL), you
may use <literal>identity</literal> key generation. For databases that support
sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you may use
<literal>sequence</literal> style key generation. Both these strategies require
two SQL queries to insert a new object.
</para>
<programlisting><![CDATA[<id name="id" type="long" column="person_id">
<generator class="sequence">
<param name="sequence">person_id_sequence</param>
</generator>
</id>]]></programlisting>
<programlisting><![CDATA[<id name="id" type="long" column="person_id" unsaved-value="0">
<generator class="identity"/>
</id>]]></programlisting>
<para>
For cross-platform development, the <literal>native</literal> strategy will
choose from the <literal>identity</literal>, <literal>sequence</literal> and
<literal>hilo</literal> strategies, dependant upon the capabilities of the
underlying database.
</para>
</sect3>
<sect3 id="mapping-declaration-id-assigned">
<title>Assigned identifiers</title>
<para>
If you want the application to assign identifiers (as opposed to having
Hibernate generate them), you may use the <literal>assigned</literal> generator.
This special generator will use the identifier value already assigned to the
object's identifier property. This generator is used when the primary key
is a natural key instead of a surrogate key. This is the default behavior
if you do no specify a <literal>&lt;generator&gt;</literal> element.
</para>
<para>
Choosing the <literal>assigned</literal> generator makes Hibernate use
<literal>unsaved-value="undefined"</literal>, forcing Hibernate to go to
the database to determine if an instance is transient or detached, unless
there is a version or timestamp property, or you define
<literal>Interceptor.isUnsaved()</literal>.
</para>
</sect3>
<sect3 id="mapping-declaration-id-select">
<title>Primary keys assigned by triggers</title>
<para>
For legacy schemas only (Hibernate does not generate DDL with triggers).
</para>
<programlisting><![CDATA[<id name="id" type="long" column="person_id">
<generator class="select">
<param name="key">socialSecurityNumber</param>
</generator>
</id>]]></programlisting>
<para>
In the above example, there is a unique valued property named
<literal>socialSecurityNumber</literal> defined by the class, as a
natural key, and a surrogate key named <literal>person_id</literal>
whose value is generated by a trigger.
</para>
</sect3>
</sect2>
<sect2 id="mapping-declaration-compositeid" revision="3">
<title>composite-id</title>
<programlisting><![CDATA[<composite-id
name="propertyName"
class="ClassName"
mapped="true|false"
access="field|property|ClassName">
node="element-name|."
<key-property name="propertyName" type="typename" column="column_name"/>
<key-many-to-one name="propertyName class="ClassName" column="column_name"/>
......
</composite-id>]]></programlisting>
<para>
For a table with a composite key, you may map multiple properties of the class
as identifier properties. The <literal>&lt;composite-id&gt;</literal> element
accepts <literal>&lt;key-property&gt;</literal> property mappings and
<literal>&lt;key-many-to-one&gt;</literal> mappings as child elements.
</para>
<programlisting><![CDATA[<composite-id>
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>]]></programlisting>
<para>
Your persistent class <emphasis>must</emphasis> override <literal>equals()</literal>
and <literal>hashCode()</literal> to implement composite identifier equality. It must
also implements <literal>Serializable</literal>.
</para>
<para>
Unfortunately, this approach to composite identifiers means that a persistent object
is its own identifier. There is no convenient "handle" other than the object itself.
You must instantiate an instance of the persistent class itself and populate its
identifier properties before you can <literal>load()</literal> the persistent state
associated with a composite key. We call this approach an <emphasis>embedded</emphasis>
composite identifier, and discourage it for serious applications.
</para>
<para>
A second approach is what we call a <emphasis>mapped</emphasis> composite identifier,
where the identifier properties named inside the <literal>&lt;composite-id&gt;</literal>
element are duplicated on both the persistent class and a separate identifier class.
</para>
<programlisting><![CDATA[<composite-id class="MedicareId" mapped="true">
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>]]></programlisting>
<para>
In this example, both the composite identifier class, <literal>MedicareId</literal>,
and the entity class itself have properties named <literal>medicareNumber</literal>
and <literal>dependent</literal>. The identifier class must override
<literal>equals()</literal> and <literal>hashCode()</literal> and implement.
<literal>Serializable</literal>. The disadvantage of this approach is quite
obvious&mdash;code duplication.
</para>
<para>
The following attributes are used to specify a mapped composite identifier:
</para>
<itemizedlist spacing="compact">
<listitem>
<para>
<literal>mapped</literal> (optional, defaults to <literal>false</literal>):
indicates that a mapped composite identifier is used, and that the contained
property mappings refer to both the entity class and the composite identifier
class.
</para>
</listitem>
<listitem>
<para>
<literal>class</literal> (optional, but required for a mapped composite identifier):
The class used as a composite identifier.
</para>
</listitem>
</itemizedlist>
<para>
We will describe a third, even more convenient approach where the composite identifier
is implemented as a component class in <xref linkend="components-compositeid"/>. The
attributes described below apply only to this alternative approach:
</para>
<itemizedlist spacing="compact">
<listitem>
<para>
<literal>name</literal> (optional, required for this approach): A property of
component type that holds the composite identifier (see chapter 9).
</para>
</listitem>
<listitem>
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>):
The strategy Hibernate should use for accessing the property value.
</para>
</listitem>
<listitem>
<para>
<literal>class</literal> (optional - defaults to the property type determined by
reflection): The component class used as a composite identifier (see next section).
</para>
</listitem>
</itemizedlist>
<para>
This third approach, an <emphasis>identifier component</emphasis> is the one we recommend
for almost all applications.
</para>
</sect2>
<sect2 id="mapping-declaration-discriminator" revision="3">
<title>discriminator</title>
<para>
The <literal>&lt;discriminator&gt;</literal> element is required for polymorphic persistence
using the table-per-class-hierarchy mapping strategy and declares a discriminator column of the
table. The discriminator column contains marker values that tell the persistence layer what
subclass to instantiate for a particular row. A restricted set of types may be used:
<literal>string</literal>, <literal>character</literal>, <literal>integer</literal>,
<literal>byte</literal>, <literal>short</literal>, <literal>boolean</literal>,
<literal>yes_no</literal>, <literal>true_false</literal>.
</para>
<programlistingco>
<areaspec>
<area id="discriminator1" coords="2 60"/>
<area id="discriminator2" coords="3 60" />
<area id="discriminator3" coords="4 60" />
<area id="discriminator4" coords="5 60" />
<area id="discriminator5" coords="6 60" />
</areaspec>
<programlisting><![CDATA[<discriminator
column="discriminator_column"
type="discriminator_type"
force="true|false"
insert="true|false"
formula="arbitrary sql expression"
/>]]></programlisting>
<calloutlist>
<callout arearefs="discriminator1">
<para>
<literal>column</literal> (optional - defaults to <literal>class</literal>) the
name of the discriminator column.
</para>
</callout>
<callout arearefs="discriminator2">
<para>
<literal>type</literal> (optional - defaults to <literal>string</literal>) a
name that indicates the Hibernate type
</para>
</callout>
<callout arearefs="discriminator3">
<para>
<literal>force</literal> (optional - defaults to <literal>false</literal>)
"force" Hibernate to specify allowed discriminator values even when retrieving
all instances of the root class.
</para>
</callout>
<callout arearefs="discriminator4">
<para>
<literal>insert</literal> (optional - defaults to <literal>true</literal>)
set this to <literal>false</literal> if your discriminator column is also part
of a mapped composite identifier. (Tells Hibernate to not include the column
in SQL <literal>INSERT</literal>s.)
</para>
</callout>
<callout arearefs="discriminator5">
<para>
<literal>formula</literal> (optional) an arbitrary SQL expression that is
executed when a type has to be evaluated. Allows content-based discrimination.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
Actual values of the discriminator column are specified by the
<literal>discriminator-value</literal> attribute of the <literal>&lt;class&gt;</literal> and
<literal>&lt;subclass&gt;</literal> elements.
</para>
<para>
The <literal>force</literal> attribute is (only) useful if the table contains rows with
"extra" discriminator values that are not mapped to a persistent class. This will not
usually be the case.
</para>
<para>
Using the <literal>formula</literal> attribute you can declare an arbitrary SQL expression
that will be used to evaluate the type of a row:
</para>
<programlisting><![CDATA[<discriminator
formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end"
type="integer"/>]]></programlisting>
</sect2>
<sect2 id="mapping-declaration-version" revision="1">
<title>version (optional)</title>
<para>
The <literal>&lt;version&gt;</literal> element is optional and indicates that
the table contains versioned data. This is particularly useful if you plan to
use <emphasis>long transactions</emphasis> (see below).
</para>
<programlistingco>
<areaspec>
<area id="version1" coords="2 70"/>
<area id="version2" coords="3 70"/>
<area id="version3" coords="4 70"/>
<area id="version4" coords="5 70"/>
<area id="version5" coords="6 70"/>
</areaspec>
<programlisting><![CDATA[<version
column="version_column"
name="propertyName"
type="typename"
access="field|property|ClassName"
unsaved-value="null|negative|undefined"
node="element-name|@attribute-name|element/@attribute|."
/>]]></programlisting>
<calloutlist>
<callout arearefs="version1">
<para>
<literal>column</literal> (optional - defaults to the property name): The name
of the column holding the version number.
</para>
</callout>
<callout arearefs="version2">
<para>
<literal>name</literal>: The name of a property of the persistent class.
</para>
</callout>
<callout arearefs="version3">
<para>
<literal>type</literal> (optional - defaults to <literal>integer</literal>):
The type of the version number.
</para>
</callout>
<callout arearefs="version4">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy Hibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="version5">
<para>
<literal>unsaved-value</literal> (optional - defaults to <literal>undefined</literal>):
A version property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from detached instances that were saved or loaded
in a previous session. (<literal>undefined</literal> specifies that the identifier
property value should be used.)
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
Version numbers may be of Hibernate type <literal>long</literal>, <literal>integer</literal>,
<literal>short</literal>, <literal>timestamp</literal> or <literal>calendar</literal>.
</para>
<para>
A version or timestamp property should never be null for a detached instance, so
Hibernate will detact any instance with a null version or timestamp as transient,
no matter what other <literal>unsaved-value</literal> strategies are specified.
<emphasis>Declaring a nullable version or timestamp property is an easy way to avoid
any problems with transitive reattachment in Hibernate, especially useful for people
using assigned identifiers or composite keys!</emphasis>
</para>
</sect2>
<sect2 id="mapping-declaration-timestamp">
<title>timestamp (optional)</title>
<para>
The optional <literal>&lt;timestamp&gt;</literal> element indicates that the table contains
timestamped data. This is intended as an alternative to versioning. Timestamps are by nature
a less safe implementation of optimistic locking. However, sometimes the application might
use the timestamps in other ways.
</para>
<programlistingco>
<areaspec>
<area id="timestamp1" coords="2 70"/>
<area id="timestamp2" coords="3 70" />
<area id="timestamp3" coords="4 70" />
<area id="timestamp4" coords="5 70" />
</areaspec>
<programlisting><![CDATA[<timestamp
column="timestamp_column"
name="propertyName"
access="field|property|ClassName"
unsaved-value="null|undefined"
node="element-name|@attribute-name|element/@attribute|."
/>]]></programlisting>
<calloutlist>
<callout arearefs="timestamp1">
<para>
<literal>column</literal> (optional - defaults to the property name): The name
of a column holding the timestamp.
</para>
</callout>
<callout arearefs="timestamp2">
<para>
<literal>name</literal>: The name of a JavaBeans style property of
Java type <literal>Date</literal> or <literal>Timestamp</literal> of the
persistent class.
</para>
</callout>
<callout arearefs="timestamp3">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy Hibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="timestamp4">
<para>
<literal>unsaved-value</literal> (optional - defaults to <literal>null</literal>):
A version property value that indicates that an instance is newly instantiated
(unsaved), distinguishing it from detached instances that were saved or loaded
in a previous session. (<literal>undefined</literal> specifies that the identifier
property value should be used.)
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
Note that <literal>&lt;timestamp&gt;</literal> is equivalent to
<literal>&lt;version type="timestamp"&gt;</literal>.
</para>
</sect2>
<sect2 id="mapping-declaration-property" revision="2">
<title>property</title>
<para>
The <literal>&lt;property&gt;</literal> element declares a persistent, JavaBean style
property of the class.
</para>
<programlistingco>
<areaspec>
<area id="property1" coords="2 70"/>
<area id="property2" coords="3 70"/>
<area id="property3" coords="4 70"/>
<areaset id="property4-5" coords="">
<area id="property4" coords='5 70'/>
<area id="property5" coords='6 70'/>
</areaset>
<area id="property6" coords="7 70"/>
<area id="property7" coords="8 70"/>
<area id="property8" coords="9 70"/>
<area id="property9" coords="10 70"/>
<area id="property10" coords="11 70"/>
<area id="property11" coords="12 70"/>
</areaspec>
<programlisting><![CDATA[<property
name="propertyName"
column="column_name"
type="typename"
update="true|false"
insert="true|false"
formula="arbitrary SQL expression"
access="field|property|ClassName"
lazy="true|false"
unique="true|false"
not-null="true|false"
optimistic-lock="true|false"
node="element-name|@attribute-name|element/@attribute|."
index="index_name"
unique_key="unique_key_id"
length="L"
precision="P"
scale="S"
/>]]></programlisting>
<calloutlist>
<callout arearefs="property1">
<para>
<literal>name</literal>: the name of the property, with an initial lowercase
letter.
</para>
</callout>
<callout arearefs="property2">
<para>
<literal>column</literal> (optional - defaults to the property name): the name
of the mapped database table column. This may also be specified by nested
<literal>&lt;column&gt;</literal> element(s).
</para>
</callout>
<callout arearefs="property3">
<para>
<literal>type</literal> (optional): a name that indicates the Hibernate type.
</para>
</callout>
<callout arearefs="property4-5">
<para>
<literal>update, insert</literal> (optional - defaults to <literal>true</literal>) :
specifies that the mapped columns should be included in SQL <literal>UPDATE</literal>
and/or <literal>INSERT</literal> statements. Setting both to <literal>false</literal>
allows a pure "derived" property whose value is initialized from some other
property that maps to the same colum(s) or by a trigger or other application.
</para>
</callout>
<callout arearefs="property6">
<para>
<literal>formula</literal> (optional): an SQL expression that defines the value for a
<emphasis>computed</emphasis> property. Computed properties do not have a column
mapping of their own.
</para>
</callout>
<callout arearefs="property7">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy Hibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="property8">
<para>
<literal>lazy</literal> (optional - defaults to <literal>false</literal>): Specifies
that this property should be fetched lazily when the instance variable is first
accessed (requires build-time bytecode instrumentation).
</para>
</callout>
<callout arearefs="property9">
<para>
<literal>unique</literal> (optional): Enable the DDL generation of a unique
constraint for the columns. Also, allow this to be the target of
a <literal>property-ref</literal>.
</para>
</callout>
<callout arearefs="property10">
<para>
<literal>not-null</literal> (optional): Enable the DDL generation of a nullability
constraint for the columns.
</para>
</callout>
<callout arearefs="property11">
<para>
<literal>optimistic-lock</literal> (optional - defaults to <literal>true</literal>):
Specifies that updates to this property do or do not require acquisition of the
optimistic lock. In other words, determines if a version increment should occur when
this property is dirty.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
<emphasis>typename</emphasis> could be:
</para>
<orderedlist spacing="compact">
<listitem>
<para>
The name of a Hibernate basic type (eg. <literal>integer, string, character,
date, timestamp, float, binary, serializable, object, blob</literal>).
</para>
</listitem>
<listitem>
<para>
The name of a Java class with a default basic type (eg. <literal>int, float,
char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob</literal>).
</para>
</listitem>
<listitem>
<para>
The name of a serializable Java class.
</para>
</listitem>
<listitem>
<para>
The class name of a custom type (eg. <literal>com.illflow.type.MyCustomType</literal>).
</para>
</listitem>
</orderedlist>
<para>
If you do not specify a type, Hibernate will use reflection upon the named
property to take a guess at the correct Hibernate type. Hibernate will try to
interpret the name of the return class of the property getter using rules 2, 3,
4 in that order. However, this is not always enough.
In certain cases you will still need the <literal>type</literal>
attribute. (For example, to distinguish between <literal>Hibernate.DATE</literal> and
<literal>Hibernate.TIMESTAMP</literal>, or to specify a custom type.)
</para>
<para>
The <literal>access</literal> attribute lets you control how Hibernate will access
the property at runtime. By default, Hibernate will call the property get/set pair.
If you specify <literal>access="field"</literal>, Hibernate will bypass the get/set
pair and access the field directly, using reflection. You may specify your own
strategy for property access by naming a class that implements the interface
<literal>org.hibernate.property.PropertyAccessor</literal>.
</para>
<para>
An especially powerful feature are derived properties. These properties are by
definition read-only, the property value is computed at load time. You declare
the computation as a SQL expression, this translates to a <literal>SELECT</literal>
clause subquery in the SQL query that loads an instance:
</para>
<programlisting><![CDATA[
<property name="totalPrice"
formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product p
WHERE li.productId = p.productId
AND li.customerId = customerId
AND li.orderNumber = orderNumber )"/>]]></programlisting>
<para>
Note that you can reference the entities own table by not declaring an alias on
a particular column (<literal>customerId</literal> in the given example). Also note
that you can use the nested <literal>&lt;formula&gt;</literal> mapping element
if you don't like to use the attribute.
</para>
</sect2>
<sect2 id="mapping-declaration-manytoone" revision="5">
<title>many-to-one</title>
<para>
An ordinary association to another persistent class is declared using a
<literal>many-to-one</literal> element. The relational model is a
many-to-one association: a foreign key in one table is referencing
the primary key column(s) of the target table.
</para>
<programlistingco>
<areaspec>
<area id="manytoone1" coords="2 70"/>
<area id="manytoone2" coords="3 70"/>
<area id="manytoone3" coords="4 70"/>
<area id="manytoone4" coords="5 70"/>
<area id="manytoone5" coords="6 70"/>
<areaset id="manytoone6-7" coords="">
<area id="manytoone6" coords='7 70'/>
<area id="manytoone7" coords='8 70'/>
</areaset>
<area id="manytoone8" coords="9 70"/>
<area id="manytoone9" coords="10 70"/>
<area id="manytoone10" coords="11 70"/>
<area id="manytoone11" coords="12 70"/>
<area id="manytoone12" coords="13 70"/>
<area id="manytoone13" coords="14 70"/>
<area id="manytoone14" coords="15 70"/>
<area id="manytoone15" coords="16 70"/>
<area id="manytoone16" coords="17 70"/>
</areaspec>
<programlisting><![CDATA[<many-to-one
name="propertyName"
column="column_name"
class="ClassName"
cascade="cascade_style"
fetch="join|select"
update="true|false"
insert="true|false"
property-ref="propertyNameFromAssociatedClass"
access="field|property|ClassName"
unique="true|false"
not-null="true|false"
optimistic-lock="true|false"
lazy="proxy|no-proxy|false"
not-found="ignore|exception"
entity-name="EntityName"
formul="arbitrary SQL expression"
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
index="index_name"
unique_key="unique_key_id"
foreign-key="foreign_key_name"
/>]]></programlisting>
<calloutlist>
<callout arearefs="manytoone1">
<para>
<literal>name</literal>: The name of the property.
</para>
</callout>
<callout arearefs="manytoone2">
<para>
<literal>column</literal> (optional): The name of the foreign key column.
This may also be specified by nested <literal>&lt;column&gt;</literal>
element(s).
</para>
</callout>
<callout arearefs="manytoone3">
<para>
<literal>class</literal> (optional - defaults to the property type
determined by reflection): The name of the associated class.
</para>
</callout>
<callout arearefs="manytoone4">
<para>
<literal>cascade</literal> (optional): Specifies which operations should
be cascaded from the parent object to the associated object.
</para>
</callout>
<callout arearefs="manytoone5">
<para>
<literal>fetch</literal> (optional - defaults to <literal>select</literal>):
Chooses between outer-join fetching or sequential select fetching.
</para>
</callout>
<callout arearefs="manytoone6-7">
<para>
<literal>update, insert</literal> (optional - defaults to <literal>true</literal>)
specifies that the mapped columns should be included in SQL <literal>UPDATE</literal>
and/or <literal>INSERT</literal> statements. Setting both to <literal>false</literal>
allows a pure "derived" association whose value is initialized from some other
property that maps to the same colum(s) or by a trigger or other application.
</para>
</callout>
<callout arearefs="manytoone8">
<para>
<literal>property-ref</literal>: (optional) The name of a property of the associated
class that is joined to this foreign key. If not specified, the primary key of
the associated class is used.
</para>
</callout>
<callout arearefs="manytoone9">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy Hibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="manytoone10">
<para>
<literal>unique</literal> (optional): Enable the DDL generation of a unique
constraint for the foreign-key column. Also, allow this to be the target of
a <literal>property-ref</literal>. This makes the association multiplicity
effectively one to one.
</para>
</callout>
<callout arearefs="manytoone11">
<para>
<literal>not-null</literal> (optional): Enable the DDL generation of a nullability
constraint for the foreign key columns.
</para>
</callout>
<callout arearefs="manytoone12">
<para>
<literal>optimistic-lock</literal> (optional - defaults to <literal>true</literal>):
Specifies that updates to this property do or do not require acquisition of the
optimistic lock. In other words, dertermines if a version increment should occur when
this property is dirty.
</para>
</callout>
<callout arearefs="manytoone13">
<para>
<literal>lazy</literal> (optional - defaults to <literal>proxy</literal>):
By default, single point associations are proxied. <literal>lazy="no-proxy"</literal>
specifies that the property should be fetched lazily when the instance variable
is first accessed (requires build-time bytecode instrumentation).
<literal>lazy="false"</literal> specifies that the association will always
be eagerly fetched.
</para>
</callout>
<callout arearefs="manytoone14">
<para>
<literal>not-found</literal> (optional - defaults to <literal>exception</literal>):
Specifies how foreign keys that reference missing rows will be handled:
<literal>ignore</literal> will treat a missing row as a null association.
</para>
</callout>
<callout arearefs="manytoone15">
<para>
<literal>entity-name</literal> (optional): The entity name of the associated class.
</para>
</callout>
</calloutlist>
<callout arearefs="manytoone16">
<para>
<literal>formula</literal> (optional): an SQL expression that defines the value for a
<emphasis>computed</emphasis> foreign key.
</para>
</callout>
</programlistingco>
<para>
Setting a value of the <literal>cascade</literal> attribute to any meaningful
value other than <literal>none</literal> will propagate certain operations to the
associated object. The meaningful values are the names of Hibernate's basic
operations, <literal>persist, merge, delete, save-update, evict, replicate, lock,
refresh</literal>, as well as the special values <literal>delete-orphan</literal>
and <literal>all</literal> and comma-separated combinations of operation
names, for example, <literal>cascade="persist,merge,evict"</literal> or
<literal>cascade="all,delete-orphan"</literal>. See <xref linkend="objectstate-transitive"/>
for a full explanation. Note that single valued associations (many-to-one and
one-to-one associations) do not support orphan delete.
</para>
<para>
A typical <literal>many-to-one</literal> declaration looks as simple as this:
</para>
<programlisting><![CDATA[<many-to-one name="product" class="Product" column="PRODUCT_ID"/>]]></programlisting>
<para>
The <literal>property-ref</literal> attribute should only be used for mapping legacy
data where a foreign key refers to a unique key of the associated table other than
the primary key. This is an ugly relational model. For example, suppose the
<literal>Product</literal> class had a unique serial number, that is not the primary
key. (The <literal>unique</literal> attribute controls Hibernate's DDL generation with
the SchemaExport tool.)
</para>
<programlisting><![CDATA[<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>]]></programlisting>
<para>
Then the mapping for <literal>OrderItem</literal> might use:
</para>
<programlisting><![CDATA[<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>]]></programlisting>
<para>
This is certainly not encouraged, however.
</para>
<para>
If the referenced unique key comprises multiple properties of the associated entity, you should
map the referenced properties inside a named <literal>&lt;properties&gt;</literal> element.
</para>
<para>
If the referenced unique key is the property of a component, you may specify a property path:
</para>
<programlisting><![CDATA[<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>]]></programlisting>
</sect2>
<sect2 id="mapping-declaration-onetoone" revision="3">
<title>one-to-one</title>
<para>
A one-to-one association to another persistent class is declared using a
<literal>one-to-one</literal> element.
</para>
<programlistingco>
<areaspec>
<area id="onetoone1" coords="2 70"/>
<area id="onetoone2" coords="3 70"/>
<area id="onetoone3" coords="4 70"/>
<area id="onetoone4" coords="5 70"/>
<area id="onetoone5" coords="6 70"/>
<area id="onetoone6" coords="7 70"/>
<area id="onetoone7" coords="8 70"/>
<area id="onetoone8" coords="9 70"/>
<area id="onetoone9" coords="10 70"/>
<area id="onetoone10" coords="11 70"/>
</areaspec>
<programlisting><![CDATA[<one-to-one
name="propertyName"
class="ClassName"
cascade="cascade_style"
constrained="true|false"
fetch="join|select"
property-ref="propertyNameFromAssociatedClass"
access="field|property|ClassName"
formula="any SQL expression"
lazy="proxy|no-proxy|false"
entity-name="EntityName"
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
foreign-key="foreign_key_name"
/>]]></programlisting>
<calloutlist>
<callout arearefs="onetoone1">
<para>
<literal>name</literal>: The name of the property.
</para>
</callout>
<callout arearefs="onetoone2">
<para>
<literal>class</literal> (optional - defaults to the property type
determined by reflection): The name of the associated class.
</para>
</callout>
<callout arearefs="onetoone3">
<para>
<literal>cascade</literal> (optional) specifies which operations should
be cascaded from the parent object to the associated object.
</para>
</callout>
<callout arearefs="onetoone4">
<para>
<literal>constrained</literal> (optional) specifies that a foreign key constraint
on the primary key of the mapped table references the table of the associated
class. This option affects the order in which <literal>save()</literal> and
<literal>delete()</literal> are cascaded, and determines whether the association
may be proxied (it is also used by the schema export tool).
</para>
</callout>
<callout arearefs="onetoone5">
<para>
<literal>fetch</literal> (optional - defaults to <literal>select</literal>):
Chooses between outer-join fetching or sequential select fetching.
</para>
</callout>
<callout arearefs="onetoone6">
<para>
<literal>property-ref</literal>: (optional) The name of a property of the associated class
that is joined to the primary key of this class. If not specified, the primary key of
the associated class is used.
</para>
</callout>
<callout arearefs="onetoone7">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy Hibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="onetoone8">
<para>
<literal>formula</literal> (optional): Almost all one to one associations map to the
primary key of the owning entity. In the rare case that this is not the case, you may
specify a some other column, columns or expression to join on using an SQL formula. (See
<literal>org.hibernate.test.onetooneformula</literal> for an example.)
</para>
</callout>
<callout arearefs="onetoone9">
<para>
<literal>lazy</literal> (optional - defaults to <literal>proxy</literal>):
By default, single point associations are proxied. <literal>lazy="no-proxy"</literal>
specifies that the property should be fetched lazily when the instance variable
is first accessed (requires build-time bytecode instrumentation).
<literal>lazy="false"</literal> specifies that the association will always
be eagerly fetched. <emphasis>Note that if <literal>constrained="false"</literal>,
proxying is impossible and Hibernate will eager fetch the association!</emphasis>
</para>
</callout>
<callout arearefs="onetoone10">
<para>
<literal>entity-name</literal> (optional): The entity name of the associated class.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
There are two varieties of one-to-one association:
</para>
<itemizedlist>
<listitem><para>
primary key associations
</para></listitem>
<listitem><para>
unique foreign key associations
</para></listitem>
</itemizedlist>
<para>
Primary key associations don't need an extra table column; if two rows are related by
the association then the two table rows share the same primary key value. So if you want
two objects to be related by a primary key association, you must make sure that they
are assigned the same identifier value!
</para>
<para>
For a primary key association, add the following mappings to <literal>Employee</literal> and
<literal>Person</literal>, respectively.
</para>
<programlisting><![CDATA[<one-to-one name="person" class="Person"/>]]></programlisting>
<programlisting><![CDATA[<one-to-one name="employee" class="Employee" constrained="true"/>]]></programlisting>
<para>
Now we must ensure that the primary keys of related rows in the PERSON and
EMPLOYEE tables are equal. We use a special Hibernate identifier generation strategy
called <literal>foreign</literal>:
</para>
<programlisting><![CDATA[<class name="person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="foreign">
<param name="property">employee</param>
</generator>
</id>
...
<one-to-one name="employee"
class="Employee"
constrained="true"/>
</class>]]></programlisting>
<para>
A newly saved instance of <literal>Person</literal> is then assigned the same primary
key value as the <literal>Employee</literal> instance refered with the <literal>employee</literal>
property of that <literal>Person</literal>.
</para>
<para>
Alternatively, a foreign key with a unique constraint, from <literal>Employee</literal> to
<literal>Person</literal>, may be expressed as:
</para>
<programlisting><![CDATA[<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>]]></programlisting>
<para>
And this association may be made bidirectional by adding the following to the
<literal>Person</literal> mapping:
</para>
<programlisting><![CDATA[<one-to-one name"employee" class="Employee" property-ref="person"/>]]></programlisting>
</sect2>
<sect2 id="mapping-declaration-naturalid">
<title>natural-id</title>
<programlisting><![CDATA[<natural-id mutable="true|false"/>
<property ... />
<many-to-one ... />
......
</natural-id>]]></programlisting>
<para>
Even though we recommend the use of surrogate keys as primary keys, you should still try
to identify natural keys for all entities. A natural key is a property or combination of
properties that is unique and non-null. If it is also immutable, even better. Map the
properties of the natural key inside the <literal>&lt;natural-id&gt;</literal> element.
Hibernate will generate the necessary unique key and nullability constraints, and your
mapping will be more self-documenting.
</para>
<para>
We strongly recommend that you implement <literal>equals()</literal> and
<literal>hashCode()</literal> to compare the natural key properties of the entity.
</para>
<para>
This mapping is not intended for use with entities with natural primary keys.
</para>
<itemizedlist spacing="compact">
<listitem>
<para>
<literal>mutable</literal> (optional, defaults to <literal>false</literal>):
By default, natural identifier properties as assumed to be immutable (constant).
</para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="mapping-declaration-component" revision="2">
<title>component, dynamic-component</title>
<para>
The <literal>&lt;component&gt;</literal> element maps properties of a
child object to columns of the table of a parent class. Components may, in
turn, declare their own properties, components or collections. See
"Components" below.
</para>
<programlistingco>
<areaspec>
<area id="component1" coords="2 45"/>
<area id="component2" coords="3 45"/>
<area id="component3" coords="4 45"/>
<area id="component4" coords="5 45"/>
<area id="component5" coords="6 45"/>
<area id="component6" coords="7 45"/>
<area id="component7" coords="8 45"/>
<area id="component8" coords="9 45"/>
</areaspec>
<programlisting><![CDATA[<component
name="propertyName"
class="className"
insert="true|false"
update="true|false"
access="field|property|ClassName"
lazy="true|false"
optimistic-lock="true|false"
unique="true|false"
node="element-name|."
>
<property ...../>
<many-to-one .... />
........
</component>]]></programlisting>
<calloutlist>
<callout arearefs="component1">
<para>
<literal>name</literal>: The name of the property.
</para>
</callout>
<callout arearefs="component2">
<para>
<literal>class</literal> (optional - defaults to the property type
determined by reflection): The name of the component (child) class.
</para>
</callout>
<callout arearefs="component3">
<para>
<literal>insert</literal>: Do the mapped columns appear in SQL
<literal>INSERT</literal>s?
</para>
</callout>
<callout arearefs="component4">
<para>
<literal>update</literal>: Do the mapped columns appear in SQL
<literal>UPDATE</literal>s?
</para>
</callout>
<callout arearefs="component5">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy Hibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="component6">
<para>
<literal>lazy</literal> (optional - defaults to <literal>false</literal>): Specifies
that this component should be fetched lazily when the instance variable is first
accessed (requires build-time bytecode instrumentation).
</para>
</callout>
<callout arearefs="component7">
<para>
<literal>optimistic-lock</literal> (optional - defaults to <literal>true</literal>):
Specifies that updates to this component do or do not require acquisition of the
optimistic lock. In other words, determines if a version increment should occur when
this property is dirty.
</para>
</callout>
<callout arearefs="component8">
<para>
<literal>unique</literal> (optional - defaults to <literal>false</literal>):
Specifies that a unique constraint exists upon all mapped columns of the
component.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
The child <literal>&lt;property&gt;</literal> tags map properties of the
child class to table columns.
</para>
<para>
The <literal>&lt;component&gt;</literal> element allows a <literal>&lt;parent&gt;</literal>
subelement that maps a property of the component class as a reference back to the
containing entity.
</para>
<para>
The <literal>&lt;dynamic-component&gt;</literal> element allows a <literal>Map</literal>
to be mapped as a component, where the property names refer to keys of the map, see
<xref linkend="components-dynamic"/>.
</para>
</sect2>
<sect2 id="mapping-declaration-properties" revision="2">
<title>properties</title>
<para>
The <literal>&lt;properties&gt;</literal> element allows the definition of a named,
logical grouping of properties of a class. The most important use of the construct
is that it allows a combination of properties to be the target of a
<literal>property-ref</literal>. It is also a convenient way to define a multi-column
unique constraint.
</para>
<programlistingco>
<areaspec>
<area id="properties1" coords="2 45"/>
<area id="properties2" coords="3 45"/>
<area id="properties3" coords="4 45"/>
<area id="properties4" coords="5 45"/>
<area id="properties5" coords="6 45"/>
</areaspec>
<programlisting><![CDATA[<properties
name="logicalName"
insert="true|false"
update="true|false"
optimistic-lock="true|false"
unique="true|false"
>
<property ...../>
<many-to-one .... />
........
</properties>]]></programlisting>
<calloutlist>
<callout arearefs="properties1">
<para>
<literal>name</literal>: The logical name of the grouping -
<emphasis>not</emphasis> an actual property name.
</para>
</callout>
<callout arearefs="properties2">
<para>
<literal>insert</literal>: Do the mapped columns appear in SQL
<literal>INSERT</literal>s?
</para>
</callout>
<callout arearefs="properties3">
<para>
<literal>update</literal>: Do the mapped columns appear in SQL
<literal>UPDATE</literal>s?
</para>
</callout>
<callout arearefs="properties4">
<para>
<literal>optimistic-lock</literal> (optional - defaults to <literal>true</literal>):
Specifies that updates to these properties do or do not require acquisition of the
optimistic lock. In other words, determines if a version increment should occur when
these properties are dirty.
</para>
</callout>
<callout arearefs="properties5">
<para>
<literal>unique</literal> (optional - defaults to <literal>false</literal>):
Specifies that a unique constraint exists upon all mapped columns of the
component.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
For example, if we have the following <literal>&lt;properties&gt;</literal> mapping:
</para>
<programlisting><![CDATA[<class name="Person">
<id name="personNumber"/>
...
<properties name="name"
unique="true" update="false">
<property name="firstName"/>
<property name="initial"/>
<property name="lastName"/>
</properties>
</class>]]></programlisting>
<para>
Then we might have some legacy data association which refers to this unique key of
the <literal>Person</literal> table, instead of to the primary key:
</para>
<programlisting><![CDATA[<many-to-one name="person"
class="Person" property-ref="name">
<column name="firstName"/>
<column name="initial"/>
<column name="lastName"/>
</many-to-one>]]></programlisting>
<para>
We don't recommend the use of this kind of thing outside the context of mapping
legacy data.
</para>
</sect2>
<sect2 id="mapping-declaration-subclass" revision="3">
<title>subclass</title>
<para>
Finally, polymorphic persistence requires the declaration of each subclass of
the root persistent class. For the table-per-class-hierarchy
mapping strategy, the <literal>&lt;subclass&gt;</literal> declaration is used.
</para>
<programlistingco>
<areaspec>
<area id="subclass1" coords="2 55"/>
<area id="subclass2" coords="3 55"/>
<area id="subclass3" coords="4 55"/>
<area id="subclass4" coords="5 55"/>
</areaspec>
<programlisting><![CDATA[<subclass
name="ClassName"
discriminator-value="discriminator_value"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
entity-name="EntityName"
node="element-name">
<property .... />
.....
</subclass>]]></programlisting>
<calloutlist>
<callout arearefs="subclass1">
<para>
<literal>name</literal>: The fully qualified class name of the subclass.
</para>
</callout>
<callout arearefs="subclass2">
<para>
<literal>discriminator-value</literal> (optional - defaults to the class name): A
value that distiguishes individual subclasses.
</para>
</callout>
<callout arearefs="subclass3">
<para>
<literal>proxy</literal> (optional): Specifies a class or interface to use for
lazy initializing proxies.
</para>
</callout>
<callout arearefs="subclass4">
<para>
<literal>lazy</literal> (optional, defaults to <literal>true</literal>): Setting
<literal>lazy="false"</literal> disables the use of lazy fetching.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
Each subclass should declare its own persistent properties and subclasses.
<literal>&lt;version&gt;</literal> and <literal>&lt;id&gt;</literal> properties
are assumed to be inherited from the root class. Each subclass in a heirarchy must
define a unique <literal>discriminator-value</literal>. If none is specified, the
fully qualified Java class name is used.
</para>
<para>
It is possible to define <literal>subclass</literal>, <literal>union-subclass</literal>,
and <literal>joined-subclass</literal> mappings in separate mapping documents, directly beneath
<literal>hibernate-mapping</literal>. This allows you to extend a class hierachy just by adding
a new mapping file. You must specify an <literal>extends</literal> attribute in the subclass mapping,
naming a previously mapped superclass. Note: Previously this feature made the ordering of the mapping
documents important. Since Hibernate3, the ordering of mapping files does not matter when using the
extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses
before subclasses.
</para>
<programlisting><![CDATA[
<hibernate-mapping>
<subclass name="DomesticCat" extends="Cat" discriminator-value="D">
<property name="name" type="string"/>
</subclass>
</hibernate-mapping>]]></programlisting>
<para>
For information about inheritance mappings, see <xref linkend="inheritance"/>.
</para>
</sect2>
<sect2 id="mapping-declaration-joinedsubclass" revision="3">
<title>joined-subclass</title>
<para>
Alternatively, each subclass may be mapped to its own table (table-per-subclass
mapping strategy). Inherited state is retrieved by joining with the table of the
superclass. We use the <literal>&lt;joined-subclass&gt;</literal> element.
</para>
<programlistingco>
<areaspec>
<area id="joinedsubclass1" coords="2 45"/>
<area id="joinedsubclass2" coords="3 45"/>
<area id="joinedsubclass3" coords="4 45"/>
<area id="joinedsubclass4" coords="5 45"/>
</areaspec>
<programlisting><![CDATA[<joined-subclass
name="ClassName"
table="tablename"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
extends="SuperclassName"
persister="ClassName"
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
<key .... >
<property .... />
.....
</joined-subclass>]]></programlisting>
<calloutlist>
<callout arearefs="joinedsubclass1">
<para>
<literal>name</literal>: The fully qualified class name of the subclass.
</para>
</callout>
<callout arearefs="joinedsubclass2">
<para>
<literal>table</literal>: The name of the subclass table.
</para>
</callout>
<callout arearefs="joinedsubclass3">
<para>
<literal>proxy</literal> (optional): Specifies a class or interface to use
for lazy initializing proxies.
</para>
</callout>
<callout arearefs="joinedsubclass4">
<para>
<literal>lazy</literal> (optional, defaults to <literal>true</literal>): Setting
<literal>lazy="false"</literal> disables the use of lazy fetching.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
No discriminator column is required for this mapping strategy. Each subclass must,
however, declare a table column holding the object identifier using the
<literal>&lt;key&gt;</literal> element. The mapping at the start of the chapter
would be re-written as:
</para>
<programlisting><![CDATA[<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class name="Cat" table="CATS">
<id name="id" column="uid" type="long">
<generator class="hilo"/>
</id>
<property name="birthdate" type="date"/>
<property name="color" not-null="true"/>
<property name="sex" not-null="true"/>
<property name="weight"/>
<many-to-one name="mate"/>
<set name="kittens">
<key column="MOTHER"/>
<one-to-many class="Cat"/>
</set>
<joined-subclass name="DomesticCat" table="DOMESTIC_CATS">
<key column="CAT"/>
<property name="name" type="string"/>
</joined-subclass>
</class>
<class name="eg.Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>]]></programlisting>
<para>
For information about inheritance mappings, see <xref linkend="inheritance"/>.
</para>
</sect2>
<sect2 id="mapping-declaration-unionsubclass" revision="2">
<title>union-subclass</title>
<para>
A third option is to map only the concrete classes of an inheritance hierarchy
to tables, (the table-per-concrete-class strategy) where each table defines all
persistent state of the class, including inherited state. In Hibernate, it is
not absolutely necessary to explicitly map such inheritance hierarchies. You
can simply map each class with a separate <literal>&lt;class&gt;</literal>
declaration. However, if you wish use polymorphic associations (e.g. an association
to the superclass of your hierarchy), you need to
use the <literal>&lt;union-subclass&gt;</literal> mapping.
</para>
<programlistingco>
<areaspec>
<area id="unionsubclass1" coords="2 45"/>
<area id="unionsubclass2" coords="3 45"/>
<area id="unionsubclass3" coords="4 45"/>
<area id="unionsubclass4" coords="5 45"/>
</areaspec>
<programlisting><![CDATA[<union-subclass
name="ClassName"
table="tablename"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
extends="SuperclassName"
abstract="true|false"
persister="ClassName"
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
<property .... />
.....
</union-subclass>]]></programlisting>
<calloutlist>
<callout arearefs="unionsubclass1">
<para>
<literal>name</literal>: The fully qualified class name of the subclass.
</para>
</callout>
<callout arearefs="unionsubclass2">
<para>
<literal>table</literal>: The name of the subclass table.
</para>
</callout>
<callout arearefs="unionsubclass3">
<para>
<literal>proxy</literal> (optional): Specifies a class or interface to use
for lazy initializing proxies.
</para>
</callout>
<callout arearefs="unionsubclass4">
<para>
<literal>lazy</literal> (optional, defaults to <literal>true</literal>): Setting
<literal>lazy="false"</literal> disables the use of lazy fetching.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
No discriminator column or key column is required for this mapping strategy.
</para>
<para>
For information about inheritance mappings, see <xref linkend="inheritance"/>.
</para>
</sect2>
<sect2 id="mapping-declaration-join" revision="3">
<title>join</title>
<para>
Using the <literal>&lt;join&gt;</literal> element, it is possible to map
properties of one class to several tables.
</para>
<programlistingco>
<areaspec>
<area id="join1" coords="2 50"/>
<area id="join2" coords="3 50"/>
<area id="join3" coords="4 50"/>
<area id="join4" coords="5 50"/>
<area id="join5" coords="6 50"/>
<area id="join6" coords="7 50"/>
</areaspec>
<programlisting><![CDATA[<join
table="tablename"
schema="owner"
catalog="catalog"
fetch="join|select"
inverse="true|false"
optional="true|false">
<key ... />
<property ... />
...
</join>]]></programlisting>
<calloutlist>
<callout arearefs="join1">
<para>
<literal>table</literal>: The name of the joined table.
</para>
</callout>
<callout arearefs="join2">
<para>
<literal>schema</literal> (optional): Override the schema name specified by
the root <literal>&lt;hibernate-mapping&gt;</literal> element.
</para>
</callout>
<callout arearefs="join3">
<para>
<literal>catalog</literal> (optional): Override the catalog name specified by
the root <literal>&lt;hibernate-mapping&gt;</literal> element.
</para>
</callout>
<callout arearefs="join4">
<para>
<literal>fetch</literal> (optional - defaults to <literal>join</literal>):
If set to <literal>join</literal>, the default, Hibernate will use an inner join
to retrieve a <literal>&lt;join&gt;</literal> defined by a class or its superclasses
and an outer join for a <literal>&lt;join&gt;</literal> defined by a subclass.
If set to <literal>select</literal> then Hibernate will use a sequential select for
a <literal>&lt;join&gt;</literal> defined on a subclass, which will be issued only
if a row turns out to represent an instance of the subclass. Inner joins will still
be used to retrieve a <literal>&lt;join&gt;</literal> defined by the class and its
superclasses.
</para>
</callout>
<callout arearefs="join5">
<para>
<literal>inverse</literal> (optional - defaults to <literal>false</literal>):
If enabled, Hibernate will not try to insert or update the properties defined
by this join.
</para>
</callout>
<callout arearefs="join6">
<para>
<literal>optional</literal> (optional - defaults to <literal>false</literal>):
If enabled, Hibernate will insert a row only if the properties defined by this
join are non-null and will always use an outer join to retrieve the properties.
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
For example, the address information for a person can be mapped to a separate
table (while preserving value type semantics for all properties):
</para>
<programlisting><![CDATA[<class name="Person"
table="PERSON">
<id name="id" column="PERSON_ID">...</id>
<join table="ADDRESS">
<key column="ADDRESS_ID"/>
<property name="address"/>
<property name="zip"/>
<property name="country"/>
</join>
...]]></programlisting>
<para>
This feature is often only useful for legacy data models, we recommend fewer
tables than classes and a fine-grained domain model. However, it is useful
for switching between inheritance mapping strategies in a single hierarchy, as
explained later.
</para>
</sect2>
<sect2 id="mapping-declaration-key">
<title>key</title>
<para>
We've seen the <literal>&lt;key&gt;</literal> element crop up a few times
now. It appears anywhere the parent mapping element defines a join to
a new table, and defines the foreign key in the joined table, that references
the primary key of the original table.
</para>
<programlistingco>
<areaspec>
<area id="key1" coords="2 50"/>
<area id="key2" coords="3 50"/>
<area id="key3" coords="4 50"/>
<area id="key4" coords="5 50"/>
<area id="key5" coords="6 50"/>
<area id="key6" coords="7 50"/>
</areaspec>
<programlisting><![CDATA[<key
column="columnname"
on-delete="noaction|cascade"
property-ref="propertyName"
not-null="true|false"
update="true|false"
unique="true|false"
/>]]></programlisting>
<calloutlist>
<callout arearefs="key1">
<para>
<literal>column</literal> (optional): The name of the foreign key column.
This may also be specified by nested <literal>&lt;column&gt;</literal>
element(s).
</para>
</callout>
<callout arearefs="key2">
<para>
<literal>on-delete</literal> (optional, defaults to <literal>noaction</literal>):
Specifies whether the foreign key constraint has database-level cascade delete
enabled.
</para>
</callout>
<callout arearefs="key3">
<para>
<literal>property-ref</literal> (optional): Specifies that the foreign key refers
to columns that are not the primary key of the orginal table. (Provided for
legacy data.)
</para>
</callout>
<callout arearefs="key4">
<para>
<literal>not-null</literal> (optional): Specifies that the foreign key columns
are not nullable (this is implied whenever the foreign key is also part of the
primary key).
</para>
</callout>
<callout arearefs="key5">
<para>
<literal>update</literal> (optional): Specifies that the foreign key should never
be updated (this is implied whenever the foreign key is also part of the primary
key).
</para>
</callout>
<callout arearefs="key6">
<para>
<literal>unique</literal> (optional): Specifies that the foreign key should have
a unique constraint (this is implied whenever the foreign key is also the primary key).
</para>
</callout>
</calloutlist>
</programlistingco>
<para>
We recommend that for systems where delete performance is important, all keys should be
defined <literal>on-delete="cascade"</literal>, and Hibernate will use a database-level
<literal>ON CASCADE DELETE</literal> constraint, instead of many individual
<literal>DELETE</literal> statements. Be aware that this feature bypasses Hibernate's
usual optimistic locking strategy for versioned data.
</para>
<para>
The <literal>not-null</literal> and <literal>update</literal> attributes are useful when
mapping a unidirectional one to many association. If you map a unidirectional one to many
to a non-nullable foreign key, you <emphasis>must</emphasis> declare the key column using
<literal>&lt;key not-null="true"&gt;</literal>.
</para>
</sect2>
<sect2 id="mapping-column" revision="3">
<title>column and formula elements</title>
<para>
Any mapping element which accepts a <literal>column</literal> attribute will alternatively
accept a <literal>&lt;column&gt;</literal> subelement. Likewise, <literal>&lt;formula&gt;</literal>
is an alternative to the <literal>formula</literal> attribute.
</para>
<programlisting><![CDATA[<column
name="column_name"
length="N"
precision="N"
scale="N"
not-null="true|false"
unique="true|false"
unique-key="multicolumn_unique_key_name"
index="index_name"
sql-type="sql_type_name"
check="SQL expression"/>]]></programlisting>
<programlisting><![CDATA[<formula>SQL expression</formula>]]></programlisting>
<para>
<literal>column</literal> and <literal>formula</literal> attributes may even be combined
within the same property or association mapping to express, for example, exotic join
conditions.
</para>
<programlisting><![CDATA[<many-to-one name="homeAddress" class="Address"
insert="false" update="false">
<column name="person_id" not-null="true" length="10"/>
<formula>'MAILING'</formula>
</many-to-one>]]></programlisting>
</sect2>
<sect2 id="mapping-declaration-import">
<title>import</title>
<para>
Suppose your application has two persistent classes with the same name, and you don't want to
specify the fully qualified (package) name in Hibernate queries. Classes may be "imported"
explicitly, rather than relying upon <literal>auto-import="true"</literal>. You may even import
classes and interfaces that are not explicitly mapped.
</para>
<programlisting><![CDATA[<import class="java.lang.Object" rename="Universe"/>]]></programlisting>
<programlistingco>
<areaspec>
<area id="import1" coords="2 40"/>
<area id="import2" coords="3 40"/>
</areaspec>
<programlisting><![CDATA[<import
class="ClassName"
rename="ShortName"
/>]]></programlisting>
<calloutlist>
<callout arearefs="import1">
<para>
<literal>class</literal>: The fully qualified class name of of any Java class.
</para>
</callout>
<callout arearefs="import2">
<para>
<literal>rename</literal> (optional - defaults to the unqualified class name):
A name that may be used in the query language.
</para>
</callout>
</calloutlist>
</programlistingco>
</sect2>
<sect2 id="mapping-types-anymapping" revision="2">
<title>any</title>
<para>
There is one further type of property mapping. The <literal>&lt;any&gt;</literal> mapping element
defines a polymorphic association to classes from multiple tables. This type of mapping always
requires more than one column. The first column holds the type of the associated entity.
The remaining columns hold the identifier. It is impossible to specify a foreign key constraint
for this kind of association, so this is most certainly not meant as the usual way of mapping
(polymorphic) associations. You should use this only in very special cases (eg. audit logs,
user session data, etc).
</para>
<para>
The <literal>meta-type</literal> attribute lets the application specify a custom type that
maps database column values to persistent classes which have identifier properties of the
type specified by <literal>id-type</literal>. You must specify the mapping from values of
the meta-type to class names.
</para>
<programlisting><![CDATA[<any name="being" id-type="long" meta-type="string">
<meta-value value="TBL_ANIMAL" class="Animal"/>
<meta-value value="TBL_HUMAN" class="Human"/>
<meta-value value="TBL_ALIEN" class="Alien"/>
<column name="table_name"/>
<column name="id"/>
</any>]]></programlisting>
<programlistingco>
<areaspec>
<area id="any1" coords="2 50"/>
<area id="any2" coords="3 50"/>
<area id="any3" coords="4 50"/>
<area id="any4" coords="5 50"/>
<area id="any5" coords="6 50"/>
<area id="any6" coords="7 50"/>
</areaspec>
<programlisting><![CDATA[<any
name="propertyName"
id-type="idtypename"
meta-type="metatypename"
cascade="cascade_style"
access="field|property|ClassName"
optimistic-lock="true|false"
>
<meta-value ... />
<meta-value ... />
.....
<column .... />
<column .... />
.....
</any>]]></programlisting>
<calloutlist>
<callout arearefs="any1">
<para>
<literal>name</literal>: the property name.
</para>
</callout>
<callout arearefs="any2">
<para>
<literal>id-type</literal>: the identifier type.
</para>
</callout>
<callout arearefs="any3">
<para>
<literal>meta-type</literal> (optional - defaults to <literal>string</literal>):
Any type that is allowed for a discriminator mapping.
</para>
</callout>
<callout arearefs="any4">
<para>
<literal>cascade</literal> (optional- defaults to <literal>none</literal>):
the cascade style.
</para>
</callout>
<callout arearefs="any5">
<para>
<literal>access</literal> (optional - defaults to <literal>property</literal>): The
strategy Hibernate should use for accessing the property value.
</para>
</callout>
<callout arearefs="any6">
<para>
<literal>optimistic-lock</literal> (optional - defaults to <literal>true</literal>):
Specifies that updates to this property do or do not require acquisition of the
optimistic lock. In other words, define if a version increment should occur if this
property is dirty.
</para>
</callout>
</calloutlist>
</programlistingco>
</sect2>
</sect1>
<sect1 id="mapping-types">
<title>Hibernate Types</title>
<sect2 id="mapping-types-entitiesvalues" revision="1">
<title>Entities and values</title>
<para>
To understand the behaviour of various Java language-level objects with respect
to the persistence service, we need to classify them into two groups:
</para>
<para>
An <emphasis>entity</emphasis> exists independently of any other objects holding
references to the entity. Contrast this with the usual Java model where an
unreferenced object is garbage collected. Entities must be explicitly saved and
deleted (except that saves and deletions may be <emphasis>cascaded</emphasis>
from a parent entity to its children). This is different from the ODMG model of
object persistence by reachablity - and corresponds more closely to how
application objects are usually used in large systems. Entities support
circular and shared references. They may also be versioned.
</para>
<para>
An entity's persistent state consists of references to other entities and
instances of <emphasis>value</emphasis> types. Values are primitives,
collections (not what's inside a collection), components and certain immutable
objects. Unlike entities, values (in particular collections and components)
<emphasis>are</emphasis> persisted and deleted by reachability. Since value
objects (and primitives) are persisted and deleted along with their containing
entity they may not be independently versioned. Values have no independent
identity, so they cannot be shared by two entities or collections.
</para>
<para>
Up until now, we've been using the term "persistent class" to refer to
entities. We will continue to do that. Strictly speaking, however, not all
user-defined classes with persistent state are entities. A
<emphasis>component</emphasis> is a user defined class with value semantics.
A Java property of type <literal>java.lang.String</literal> also has value
semantics. Given this definition, we can say that all types (classes) provided
by the JDK have value type semantics in Java, while user-defined types may
be mapped with entity or value type semantics. This decision is up to the
application developer. A good hint for an entity class in a domain model are
shared references to a single instance of that class, while composition or
aggregation usually translates to a value type.
</para>
<para>
We'll revisit both concepts throughout the documentation.
</para>
<para>
The challenge is to map the Java type system (and the developers' definition of
entities and value types) to the SQL/database type system. The bridge between
both systems is provided by Hibernate: for entities we use
<literal>&lt;class&gt;</literal>, <literal>&lt;subclass&gt;</literal> and so on.
For value types we use <literal>&lt;property&gt;</literal>,
<literal>&lt;component&gt;</literal>, etc, usually with a <literal>type</literal>
attribute. The value of this attribute is the name of a Hibernate
<emphasis>mapping type</emphasis>. Hibernate provides many mappings (for standard
JDK value types) out of the box. You can write your own mapping types and implement your
custom conversion strategies as well, as you'll see later.
</para>
<para>
All built-in Hibernate types except collections support null semantics.
</para>
</sect2>
<sect2 id="mapping-types-basictypes" revision="3">
<title>Basic value types</title>
<para>
The built-in <emphasis>basic mapping types</emphasis> may be roughly categorized into
<variablelist>
<varlistentry>
<term><literal>integer, long, short, float, double, character, byte,
boolean, yes_no, true_false</literal></term>
<listitem>
<para>
Type mappings from Java primitives or wrapper classes to appropriate
(vendor-specific) SQL column types. <literal>boolean, yes_no</literal>
and <literal>true_false</literal> are all alternative encodings for
a Java <literal>boolean</literal> or <literal>java.lang.Boolean</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>string</literal></term>
<listitem>
<para>
A type mapping from <literal>java.lang.String</literal> to
<literal>VARCHAR</literal> (or Oracle <literal>VARCHAR2</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>date, time, timestamp</literal></term>
<listitem>
<para>
Type mappings from <literal>java.util.Date</literal> and its subclasses
to SQL types <literal>DATE</literal>, <literal>TIME</literal> and
<literal>TIMESTAMP</literal> (or equivalent).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>calendar, calendar_date</literal></term>
<listitem>
<para>
Type mappings from <literal>java.util.Calendar</literal> to
SQL types <literal>TIMESTAMP</literal> and <literal>DATE</literal>
(or equivalent).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>big_decimal, big_integer</literal></term>
<listitem>
<para>
Type mappings from <literal>java.math.BigDecimal</literal> and
<literal>java.math.BigInteger</literal> to <literal>NUMERIC</literal>
(or Oracle <literal>NUMBER</literal>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>locale, timezone, currency</literal></term>
<listitem>
<para>
Type mappings from <literal>java.util.Locale</literal>,
<literal>java.util.TimeZone</literal> and
<literal>java.util.Currency</literal>
to <literal>VARCHAR</literal> (or Oracle <literal>VARCHAR2</literal>).
Instances of <literal>Locale</literal> and <literal>Currency</literal> are
mapped to their ISO codes. Instances of <literal>TimeZone</literal> are
mapped to their <literal>ID</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>class</literal></term>
<listitem>
<para>
A type mapping from <literal>java.lang.Class</literal> to
<literal>VARCHAR</literal> (or Oracle <literal>VARCHAR2</literal>).
A <literal>Class</literal> is mapped to its fully qualified name.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>binary</literal></term>
<listitem>
<para>
Maps byte arrays to an appropriate SQL binary type.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>text</literal></term>
<listitem>
<para>
Maps long Java strings to a SQL <literal>CLOB</literal> or
<literal>TEXT</literal> type.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>serializable</literal></term>
<listitem>
<para>
Maps serializable Java types to an appropriate SQL binary type. You
may also indicate the Hibernate type <literal>serializable</literal> with
the name of a serializable Java class or interface that does not default
to a basic type.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>clob, blob</literal></term>
<listitem>
<para>
Type mappings for the JDBC classes <literal>java.sql.Clob</literal> and
<literal>java.sql.Blob</literal>. These types may be inconvenient for some
applications, since the blob or clob object may not be reused outside of
a transaction. (Furthermore, driver support is patchy and inconsistent.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date,
imm_serializable, imm_binary</literal>
</term>
<listitem>
<para>
Type mappings for what are usually considered mutable Java types, where
Hibernate makes certain optimizations appropriate only for immutable
Java types, and the application treats the object as immutable. For
example, you should not call <literal>Date.setTime()</literal> for an
instance mapped as <literal>imm_timestamp</literal>. To change the
value of the property, and have that change made persistent, the
application must assign a new (nonidentical) object to the property.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Unique identifiers of entities and collections may be of any basic type except
<literal>binary</literal>, <literal>blob</literal> and <literal>clob</literal>.
(Composite identifiers are also allowed, see below.)
</para>
<para>
The basic value types have corresponding <literal>Type</literal> constants defined on
<literal>org.hibernate.Hibernate</literal>. For example, <literal>Hibernate.STRING</literal>
represents the <literal>string</literal> type.
</para>
</sect2>
<sect2 id="mapping-types-custom" revision="2">
<title>Custom value types</title>
<para>
It is relatively easy for developers to create their own value types. For example,
you might want to persist properties of type <literal>java.lang.BigInteger</literal>
to <literal>VARCHAR</literal> columns. Hibernate does not provide a built-in type
for this. But custom types are not limited to mapping a property (or collection element)
to a single table column. So, for example, you might have a Java property
<literal>getName()</literal>/<literal>setName()</literal> of type
<literal>java.lang.String</literal> that is persisted to the columns
<literal>FIRST_NAME</literal>, <literal>INITIAL</literal>, <literal>SURNAME</literal>.
</para>
<para>
To implement a custom type, implement either <literal>org.hibernate.UserType</literal>
or <literal>org.hibernate.CompositeUserType</literal> and declare properties using the
fully qualified classname of the type. Check out
<literal>org.hibernate.test.DoubleStringType</literal> to see the kind of things that
are possible.
</para>
<programlisting><![CDATA[<property name="twoStrings" type="org.hibernate.test.DoubleStringType">
<column name="first_string"/>
<column name="second_string"/>
</property>]]></programlisting>
<para>
Notice the use of <literal>&lt;column&gt;</literal> tags to map a property to multiple
columns.
</para>
<para>
The <literal>CompositeUserType</literal>, <literal>EnhancedUserType</literal>,
<literal>UserCollectionType</literal>, and <literal>UserVersionType</literal>
interfaces provide support for more specialized uses.
</para>
<para>
You may even supply parameters to a <literal>UserType</literal> in the mapping file. To
do this, your <literal>UserType</literal> must implement the
<literal>org.hibernate.usertype.ParameterizedType</literal> interface. To supply parameters
to your custom type, you can use the <literal>&lt;type&gt;</literal> element in your mapping
files.
</para>
<programlisting><![CDATA[<property name="priority">
<type name="com.mycompany.usertypes.DefaultValueIntegerType">
<param name="default">0</param>
</type>
</property>]]></programlisting>
<para>
The <literal>UserType</literal> can now retrieve the value for the parameter named
<literal>default</literal> from the <literal>Properties</literal> object passed to it.
</para>
<para>
If you use a certain <literal>UserType</literal> very often, it may be useful to define a
shorter name for it. You can do this using the <literal>&lt;typedef&gt;</literal> element.
Typedefs assign a name to a custom type, and may also contain a list of default
parameter values if the type is parameterized.
</para>
<programlisting><![CDATA[<typedef class="com.mycompany.usertypes.DefaultValueIntegerType" name="default_zero">
<param name="default">0</param>
</typedef>]]></programlisting>
<programlisting><![CDATA[<property name="priority" type="default_zero"/>]]></programlisting>
<para>
It is also possible to override the parameters supplied in a typedef on a case-by-case basis
by using type parameters on the property mapping.
</para>
<para>
Even though Hibernate's rich range of built-in types and support for components means you
will very rarely <emphasis>need</emphasis> to use a custom type, it is nevertheless
considered good form to use custom types for (non-entity) classes that occur frequently
in your application. For example, a <literal>MonetaryAmount</literal> class is a good
candidate for a <literal>CompositeUserType</literal>, even though it could easily be mapped
as a component. One motivation for this is abstraction. With a custom type, your mapping
documents would be future-proofed against possible changes in your way of representing
monetary values.
</para>
</sect2>
</sect1>
<sect1 id="mapping-entityname">
<title>Mapping a class more than once</title>
<para>
It is possible to provide more than one mapping for a particular persistent class. In this
case you must specify an <emphasis>entity name</emphasis> do disambiguate between instances
of the two mapped entities. (By default, the entity name is the same as the class name.)
Hibernate lets you specify the entity name when working with persistent objects, when writing
queries, or when mapping associations to the named entity.
</para>
<programlisting><![CDATA[<class name="Contract" table="Contracts"
entity-name="CurrentContract">
...
<set name="history" inverse="true"
order-by="effectiveEndDate desc">
<key column="currentContractId"/>
<one-to-many entity-name="HistoricalContract"/>
</set>
</class>
<class name="Contract" table="ContractHistory"
entity-name="HistoricalContract">
...
<many-to-one name="currentContract"
column="currentContractId"
entity-name="CurrentContract"/>
</class>]]></programlisting>
<para>
Notice how associations are now specified using <literal>entity-name</literal> instead of
<literal>class</literal>.
</para>
</sect1>
<sect1 id="mapping-quotedidentifiers">
<title>SQL quoted identifiers</title>
<para>
You may force Hibernate to quote an identifier in the generated SQL by enclosing the table or
column name in backticks in the mapping document. Hibernate will use the correct quotation
style for the SQL <literal>Dialect</literal> (usually double quotes, but brackets for SQL
Server and backticks for MySQL).
</para>
<programlisting><![CDATA[<class name="LineItem" table="`Line Item`">
<id name="id" column="`Item Id`"/><generator class="assigned"/></id>
<property name="itemNumber" column="`Item #`"/>
...
</class>]]></programlisting>
</sect1>
<sect1 id="mapping-alternatives">
<title>Metadata alternatives</title>
<para>
XML isn't for everyone, and so there are some alternative ways to define O/R mapping metadata in Hibernate.
</para>
<sect2 id="mapping-xdoclet">
<title>Using XDoclet markup</title>
<para>
Many Hibernate users prefer to embed mapping information directly in sourcecode using
XDoclet <literal>@hibernate.tags</literal>. We will not cover this approach in this
document, since strictly it is considered part of XDoclet. However, we include the
following example of the <literal>Cat</literal> class with XDoclet mappings.
</para>
<programlisting><![CDATA[package eg;
import java.util.Set;
import java.util.Date;
/**
* @hibernate.class
* table="CATS"
*/
public class Cat {
private Long id; // identifier
private Date birthdate;
private Cat mother;
private Set kittens
private Color color;
private char sex;
private float weight;
/*
* @hibernate.id
* generator-class="native"
* column="CAT_ID"
*/
public Long getId() {
return id;
}
private void setId(Long id) {
this.id=id;
}
/**
* @hibernate.many-to-one
* column="PARENT_ID"
*/
public Cat getMother() {
return mother;
}
void setMother(Cat mother) {
this.mother = mother;
}
/**
* @hibernate.property
* column="BIRTH_DATE"
*/
public Date getBirthdate() {
return birthdate;
}
void setBirthdate(Date date) {
birthdate = date;
}
/**
* @hibernate.property
* column="WEIGHT"
*/
public float getWeight() {
return weight;
}
void setWeight(float weight) {
this.weight = weight;
}
/**
* @hibernate.property
* column="COLOR"
* not-null="true"
*/
public Color getColor() {
return color;
}
void setColor(Color color) {
this.color = color;
}
/**
* @hibernate.set
* inverse="true"
* order-by="BIRTH_DATE"
* @hibernate.collection-key
* column="PARENT_ID"
* @hibernate.collection-one-to-many
*/
public Set getKittens() {
return kittens;
}
void setKittens(Set kittens) {
this.kittens = kittens;
}
// addKitten not needed by Hibernate
public void addKitten(Cat kitten) {
kittens.add(kitten);
}
/**
* @hibernate.property
* column="SEX"
* not-null="true"
* update="false"
*/
public char getSex() {
return sex;
}
void setSex(char sex) {
this.sex=sex;
}
}]]></programlisting>
<para>
See the Hibernate web site for more examples of XDoclet and Hibernate.
</para>
</sect2>
<sect2 id="mapping-annotations" revision="2">
<title>Using JDK 5.0 Annotations</title>
<para>
JDK 5.0 introduced XDoclet-style annotations at the language level, type-safe and
checked at compile time. This mechnism is more powerful than XDoclet annotations and
better supported by tools and IDEs. IntelliJ IDEA, for example, supports auto-completion
and syntax highlighting of JDK 5.0 annotations. The new revision of the EJB specification
(JSR-220) uses JDK 5.0 annotations as the primary metadata mechanism for entity beans.
Hibernate3 implements the <literal>EntityManager</literal> of JSR-220 (the persistence API),
support for mapping metadata is available via the <emphasis>Hibernate Annotations</emphasis>
package, as a separate download. Both EJB3 (JSR-220) and Hibernate3 metadata is supported.
</para>
<para>
This is an example of a POJO class annotated as an EJB entity bean:
</para>
<programlisting><![CDATA[@Entity(access = AccessType.FIELD)
public class Customer implements Serializable {
@Id;
Long id;
String firstName;
String lastName;
Date birthday;
@Transient
Integer age;
@Embedded
private Address homeAddress;
@OneToMany(cascade=CascadeType.ALL)
@JoinColumn(name="CUSTOMER_ID")
Set<Order> orders;
// Getter/setter and business methods
}]]></programlisting>
<para>
Note that support for JDK 5.0 Annotations (and JSR-220) is still work in progress and
not completed. Please refer to the Hibernate Annotations module for more details.
</para>
</sect2>
</sect1>
</chapter>