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HHH-5149 Reviewing the collection mapping chapter 

git-svn-id: https://svn.jboss.org/repos/hibernate/core/trunk@19859 1b8cb986-b30d-0410-93ca-fae66ebed9b2
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Hardy Ferentschik 2010-06-30 11:50:41 +00:00
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documentation/manual/src/main/docbook/en-US/content/collection_mapping.xml
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@ -113,7 +113,7 @@ kittens = cat.getKittens(); // Okay, kittens collection is a Set
this:</para>
<example id="example.collection.mapping.annotations">
<title>Collection mapping using annotations</title>
<title>Collection mapping using @OneToMany and @JoinColumn</title>
<programlisting role="JAVA">@Entity
public class Product {
@ -126,6 +126,7 @@ public class Product {
void setSerialNumber(String sn) { serialNumber = sn; }
@OneToMany
@JoinColumn(name="PART_ID")
public Set&lt;Part&gt; getParts() { return parts; }
void setParts(Set parts) { this.parts = parts; }
}
@ -137,9 +138,56 @@ public class Part {
}</programlisting>
</example>
<para>Lets have a look how collections are mapped using Hibernate mapping
files. Here the first step is to chose the right mapping element, because
it depends on the type of interface. For example, a
<para>Product describes a unidirectional relationship with Part using the
join column PART_ID. In this unidirectional one to many scenario you can
also use a join table as seen in <xref
linkend="example-one-to-many-with-join-table" />.</para>
<example id="example-one-to-many-with-join-table">
<title id="example-one-to-many-with-join-table">Collection mapping using
@OneToMany and @JoinTable</title>
<programlisting role="JAVA">@Entity
public class Product {
private String serialNumber;
private Set&lt;Part&gt; parts = new HashSet&lt;Part&gt;();
@Id
public String getSerialNumber() { return serialNumber; }
void setSerialNumber(String sn) { serialNumber = sn; }
@OneToMany
@JoinTable(
name="PRODUCT_PARTS",
joinColumns = @JoinColumn( name="PRODUCT_ID"),
inverseJoinColumns = @JoinColumn( name="PART_ID")
)
public Set&lt;Part&gt; getParts() { return parts; }
void setParts(Set parts) { this.parts = parts; }
}
@Entity
public class Part {
...
}</programlisting>
</example>
<para>Without describing any physical mapping (no
<classname>@JoinColumn</classname> or <classname>@JoinTable</classname>),
a unidirectional one to many with join table is used. The table name is
the concatenation of the owner table name, _, and the other side table
name. The foreign key name(s) referencing the owner table is the
concatenation of the owner table, _, and the owner primary key column(s)
name. The foreign key name(s) referencing the other side is the
concatenation of the owner property name, _, and the other side primary
key column(s) name. A unique constraint is added to the foreign key
referencing the other side table to reflect the one to many.</para>
<para>Lets have a look now how collections are mapped using Hibernate
mapping files. In this case the first step is to chose the right mapping
element. It depends on the type of interface. For example, a
<literal>&lt;set&gt;</literal> element is used for mapping properties of
type <literal>Set</literal>.</para>
@ -438,6 +486,10 @@ public class Part {
<section id="collections-indexed">
<title id="section.indexed.collections">Indexed collections</title>
<para>In the following paragraphs we have a closer at the indexed
collections <classname>List</classname> and <classname>Map</classname>
how the their index can be mapped in Hibernate.</para>
<section>
<title>Lists</title>
@ -458,7 +510,7 @@ public class Part {
<para>To order lists in memory, add
<literal>@javax.persistence.OrderBy</literal> to your property. This
annotation takes as parameter a list of comma separated properties (of
the target entity) and order the collection accordingly (eg
the target entity) and orders the collection accordingly (eg
<code>firstname asc, age desc</code>), if the string is empty, the
collection will be ordered by the primary key of the target
entity.</para>
@ -516,7 +568,7 @@ public class Order {
<literal>orders_ORDER</literal>).</para>
<example>
<title>Additional index column using
<title>Explicit index column using
<classname>@OrderColumn</classname></title>
<programlisting language="JAVA" role="JAVA">@Entity
@ -569,11 +621,11 @@ public class Order {
is 0 like in Java.</para>
</note>
<para>Using mapping files the index of an array or list is always of
type <literal>integer</literal> and is mapped using the
<literal>&lt;list-index&gt;</literal> element. The mapped column
contains sequential integers that are numbered from zero by
default.</para>
<para>Looking again at the Hibernate mapping file equivalent, the
index of an array or list is always of type <literal>integer</literal>
and is mapped using the <literal>&lt;list-index&gt;</literal> element.
The mapped column contains sequential integers that are numbered from
zero by default.</para>
<example>
<title>index-list element for indexed collections in xml
@ -615,20 +667,21 @@ public class Order {
<section>
<title>Maps</title>
<para>Maps can borrow their keys from one of the associated entity
properties or have dedicated columns to store an explicit key.</para>
<para>The question with <classname>Map</classname>s is where the key
value is stored. There are everal options. Maps can borrow their keys
from one of the associated entity properties or have dedicated columns
to store an explicit key.</para>
<para>To use one of the target entity property as a key of the map,
use <literal>@MapKey(name="myProperty")</literal>
(<literal>myProperty</literal> is a property name in the target
entity). When using <literal>@MapKey</literal> (without property
name), the target entity primary key is used. The map key uses the
same column as the property pointed out: there is no additional column
defined to hold the map key, and it does make sense since the map key
actually represent a target property. Be aware that once loaded, the
key is no longer kept in sync with the property, in other words, if
you change the property value, the key will not change automatically
in your Java model.</para>
use <literal>@MapKey(name="myProperty")</literal>, where
<literal>myProperty</literal> is a property name in the target entity.
When using <literal>@MapKey</literal> without the name attribuate, the
target entity primary key is used. The map key uses the same column as
the property pointed out. There is no additional column defined to
hold the map key, because the map key represent a target property. Be
aware that once loaded, the key is no longer kept in sync with the
property. In other words, if you change the property value, the key
will not change automatically in your Java model.</para>
<example>
<title>Use of target entity property as map key via
@ -673,14 +726,14 @@ public class Order {
|-------------|</programlisting>
</example>
<para>Otherwise, the map key is mapped to a dedicated column or
<para>Alternatively the map key is mapped to a dedicated column or
columns. In order to customize the mapping use one of the following
annotations:</para>
<itemizedlist>
<listitem>
<para>@<classname>MapKeyColumn</classname> if the map key is a
basic type, if you don't specify the column name, the name of the
basic type. If you don't specify the column name, the name of the
property followed by underscore followed by <literal>KEY</literal>
is used (for example <literal>orders_KEY</literal>).</para>
</listitem>
@ -757,11 +810,11 @@ public class Order {
the new standard approach described above</para>
</note>
<para>Using Hibernate mapping files there exists similar concepts
using <literal>&lt;map-key&gt;</literal>,
<para>Using Hibernate mapping files there exists equivalent concepts
to the descibed annotations. You have to use
<literal>&lt;map-key&gt;</literal>,
<literal>&lt;map-key-many-to-many&gt;</literal> and
<literal>&lt;composite-map-key&gt;</literal> which are even a little
more flexible since SQL expressions can be used to define the map key.
<literal>&lt;composite-map-key&gt;</literal>.
<literal>&lt;map-key&gt;</literal> is used for any basic type,
<literal>&lt;map-key-many-to-many&gt;</literal> for an entity
reference and <literal>&lt;composite-map-key&gt;</literal> for a
@ -845,11 +898,11 @@ public class Order {
</section>
<section id="collections-ofvalues" revision="2">
<title>Collections of values</title>
<title>Collections of basic types and embeddable objects</title>
<para>In some situations you don't need to associate two entities but
simply create a collection of basic types or embeddable objects. Use the
<classname>@ElementCollection</classname> in this case.</para>
<classname>@ElementCollection</classname> for this case.</para>
<example>
<title>Collection of basic types mapped via
@ -869,7 +922,7 @@ public class User {
<para>The collection table holding the collection data is set using the
<classname>@CollectionTable</classname> annotation. If omitted the
collection table name default to the concatenation of the name of the
collection table name defaults to the concatenation of the name of the
containing entity and the name of the collection attribute, separated by
an underscore. In our example, it would be
<literal>User_nicknames</literal>.</para>
@ -992,9 +1045,9 @@ public class User {
<para>Hibernate supports collections implementing
<literal>java.util.SortedMap</literal> and
<literal>java.util.SortedSet</literal>. With annotations you declare a
sort comparator using <literal>@Sort</literal>. You between the
sort comparator using <literal>@Sort</literal>. You chose between the
comparator types unsorted, natural or custom. If you want to use your
own comparator implementation, you'll also have to express the
own comparator implementation, you'll also have to specify the
implementation class using the <literal>comparator</literal> attribute.
Note that you need to use either a <classname>SortedSet</classname> or a
<classname>SortedMap</classname> interface.</para>
@ -1005,8 +1058,6 @@ public class User {
<programlisting language="JAVA" role="JAVA">@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
@JoinColumn(name="CUST_ID")
@Sort(type = SortType.COMPARATOR, comparator = TicketComparator.class)
@Where(clause="1=1")
@OnDelete(action=OnDeleteAction.CASCADE)
public SortedSet&lt;Ticket&gt; getTickets() {
return tickets;
}</programlisting>
@ -1036,17 +1087,18 @@ public SortedSet&lt;Ticket&gt; getTickets() {
<literal>unsorted</literal>, <literal>natural</literal> and the name of
a class implementing <literal>java.util.Comparator</literal>.</para>
<para>Sorted collections actually behave like
<literal>java.util.TreeSet</literal> or
<literal>java.util.TreeMap</literal>.</para>
<tip>
<para>Sorted collections actually behave like
<literal>java.util.TreeSet</literal> or
<literal>java.util.TreeMap</literal>.</para>
</tip>
<para>If you want the database itself to order the collection elements,
use the <literal>order-by</literal> attribute of <literal>set</literal>,
<literal>bag</literal> or <literal>map</literal> mappings. This solution
is only available under JDK 1.4 or higher and is implemented using
<literal>LinkedHashSet</literal> or <literal>LinkedHashMap</literal>.
This performs the ordering in the SQL query and not in the
memory.</para>
is implemented using <literal>LinkedHashSet</literal> or
<literal>LinkedHashMap</literal> and performs the ordering in the SQL
query and not in the memory.</para>
<example>
<title>Sorting in database using order-by</title>
@ -1161,10 +1213,11 @@ public class Soldier {
}</programlisting>
</example>
<para>Again a look at the maping file approach. There you can define a
bidirectional one-to-many association by mapping a one-to-many
association to the same table column(s) as a many-to-one association and
declaring the many-valued end <literal>inverse="true"</literal>.</para>
<para>How does the mappping of a bidirectional mapping look like in
Hibernate mapping xml? There you define a bidirectional one-to-many
association by mapping a one-to-many association to the same table
column(s) as a many-to-one association and declaring the many-valued end
<literal>inverse="true"</literal>.</para>
<example>
<title>Bidirectional one to many via Hibernate mapping files</title>
@ -1233,9 +1286,7 @@ public class Employee implements Serializable {
} </programlisting>
</example>
<para>We've already shown the many declarations and the detailed
attributes for associations. We'll go deeper in the
<literal>@JoinTable</literal> description, it defines a
<para>In this example <classname>@JoinTable</classname> defines a
<literal>name</literal>, an array of join columns, and an array of
inverse join columns. The latter ones are the columns of the association
table which refer to the <classname>Employee</classname> primary key
@ -1323,11 +1374,12 @@ public class Customer {
<para>You cannot select an indexed collection.</para>
</note></para>
<para>Here is an example of a bidirectional many-to-many association
that illustrates how each category can have many items and each item can
be in many categories:</para>
<para><xref linkend="example-many-to-many-mapping-file" /> shows a
bidirectional many-to-many association that illustrates how each
category can have many items and each item can be in many
categories:</para>
<example>
<example id="example-many-to-many-mapping-file">
<title>Many to many association using Hibernate mapping files</title>
<programlisting role="XML">&lt;class name="Category"&gt;
@ -1376,15 +1428,11 @@ session.persist(category); // The relationship will be saved</prog
<section id="collections-indexedbidirectional">
<title>Bidirectional associations with indexed collections</title>
<para>Here are some additional consideration for bidirectional mappings
with indexed collections using Hibernate mapping files. Refer also to
<xref linkend="section.indexed.collections" /> and <xref
linkend="section.sorted.collections" />.</para>
<para>A bidirectional association where one end is represented as a
<literal>&lt;list&gt;</literal> or <literal>&lt;map&gt;</literal>,
requires special consideration. If there is a property of the child
class that maps to the index column you can use
<para>There are some additional considerations for bidirectional
mappings with indexed collections (where one end is represented as a
<literal>&lt;list&gt;</literal> or <literal>&lt;map&gt;</literal>) when
using Hibernate mapping files. If there is a property of the child class
that maps to the index column you can use
<literal>inverse="true"</literal> on the collection mapping:</para>
<example>
@ -1491,12 +1539,12 @@ public class Company {
<para>The majority of the many-to-many associations and collections of
values shown previously all map to tables with composite keys, even
though it has been have suggested that entities should have synthetic
though it has been suggested that entities should have synthetic
identifiers (surrogate keys). A pure association table does not seem to
benefit much from a surrogate key, although a collection of composite
values <emphasis>might</emphasis>. It is for this reason Hibernate
provides a feature that allows you to map many-to-many associations and
collections of values to a table with a surrogate key.</para>
values <emphasis>might</emphasis>. For this reason Hibernate provides a
feature that allows you to map many-to-many associations and collections
of values to a table with a surrogate key.</para>
<para>The <literal>&lt;idbag&gt;</literal> element lets you map a
<literal>List</literal> (or <literal>Collection</literal>) with bag