join column mappings
This commit is contained in:
Gavin
Christian Beikov
parent
da2542a01c
commit
7e9081b448
|
|
@ -151,7 +151,7 @@ is a bad idea, since it's impossible to create a foreign key constraint that tar
|
|||
// However, it's possible to emulate a mix of `SINGLE_TABLE` and `JOINED` inheritance using the `@SecondaryTable` annotation.
|
||||
|
||||
[[table-mappings]]
|
||||
=== Mapping to tables
|
||||
=== Mapping entities to tables
|
||||
|
||||
The following annotations specify exactly how elements of the domain model map to tables of the relational model:
|
||||
|
||||
|
|
@ -198,3 +198,149 @@ These annotations specify how elements of the domain model map to columns of tab
|
|||
| `@MapKeyColumn` | Specified a column that should be used to persist the keys of a `Map`.
|
||||
|===
|
||||
|
||||
We use the `@Column` annotation to map basic attributes.
|
||||
|
||||
[[regular-column-mappings]]
|
||||
=== Mapping basic attributes to columns
|
||||
|
||||
The `@Column` annotation is not only useful for specifying the column name.
|
||||
|
||||
|===
|
||||
| Annotation member | Purpose
|
||||
|
||||
| `name` | The name of the mapped column
|
||||
| `table` | The name of the table to which this column belongs
|
||||
| `length` | The length of a `VARCHAR`, `CHAR`, or `VARBINARY` column type
|
||||
| `precision` | The decimal digits of precision of a `FLOAT`, `DECIMAL`, `NUMERIC`, or `TIME`, or `TIMESTAMP` column type
|
||||
| `scale` | The scale of a `DECIMAL` or `NUMERIC` column type, the digits of precision that occur to the right of the decimal point
|
||||
| `unique` | Whether the column has a `UNIQUE` constraint
|
||||
| `nullable` | Whether the column has a `NOT NULL` constraint
|
||||
| `insertable` | Whether the column should appear in generated SQL `INSERT` and `UPDATE` statements
|
||||
| `updatable` | Whether the column should appear in generated SQL `INSERT` and `UPDATE` statements
|
||||
| `columnDefinition` 💀| A DDL fragment that should be used to declare the column
|
||||
|===
|
||||
|
||||
[TIP]
|
||||
.Use of `columnDefinition` results in unportable DDL
|
||||
====
|
||||
We no longer recommend the use of `columnDefinition`.
|
||||
Hibernate has much better ways to customize the generated DDL using techniques that result in portable behavior across different databases.
|
||||
====
|
||||
|
||||
Here we see four different ways to use the `@Column` annotation:
|
||||
|
||||
[source,java]
|
||||
----
|
||||
@Entity
|
||||
@Table(name="Books")
|
||||
@SecondaryTable(name="Editions")
|
||||
class Book {
|
||||
@Id @GeneratedValue
|
||||
@Column(name="bookId") // customize column name
|
||||
Long id;
|
||||
|
||||
@Column(length=100, nullable=false) // declare column as VARCHAR(100) NOT NULL
|
||||
String title;
|
||||
|
||||
@Column(length=17, unique=true, nullable=false) // declare column as VARCHAR(17) NOT NULL UNIQUE
|
||||
String isbn;
|
||||
|
||||
@Column(table="Editions", updatable=false) // column belongs to the secondary table, and is never updated
|
||||
int edition;
|
||||
}
|
||||
----
|
||||
|
||||
We don't use `@Column` to map associations.
|
||||
|
||||
[[join-column-mappings]]
|
||||
=== Mapping associations to foreign key columns
|
||||
|
||||
The `@JoinColumn` annotation is used to customize a foreign key column.
|
||||
|
||||
|===
|
||||
| Annotation member | Purpose
|
||||
|
||||
| `name` | The name of the mapped foreign key column
|
||||
| `table` | The name of the table to which this column belongs
|
||||
| `referencedColumnName` | The name of the column to which the mapped foreign key column refers
|
||||
| `unique` | Whether the column has a `UNIQUE` constraint
|
||||
| `nullable` | Whether the column has a `NOT NULL` constraint
|
||||
| `insertable` | Whether the column should appear in generated SQL `INSERT` and `UPDATE` statements
|
||||
| `updatable` | Whether the column should appear in generated SQL `INSERT` and `UPDATE` statements
|
||||
| `columnDefinition` 💀| A DDL fragment that should be used to declare the column
|
||||
| `foreignKey` | A `@ForeignKey` annotation specifying the name of the `FOREIGN KEY` constraint
|
||||
|===
|
||||
|
||||
Here we see how to use `@JoinColumn` to define a `@ManyToOne` association which mapping a foreign key column which refers to a non-primary-key `UNIQUE` column of the referenced table.
|
||||
|
||||
[source,java]
|
||||
----
|
||||
@Entity
|
||||
@Table(name="Items")
|
||||
class Item {
|
||||
...
|
||||
@ManyToOne(optional=false) // implies nullable=false
|
||||
@JoinColumn(name = "bookIsbn", referencedColumnName = "isbn", // a reference to a non-PK column
|
||||
foreignKey = @ForeignKey(name="ItemsToBooksBySsn")) // supply a name for the FOREIGN KEY constraint
|
||||
Book book;
|
||||
...
|
||||
}
|
||||
----
|
||||
|
||||
[TIP]
|
||||
.Foreign key constraint names
|
||||
====
|
||||
Notice the use of `@ForeignKey` to customize the name of the foreign key constraint.
|
||||
If you don't supply a name explicitly, Hibernate will generate a quite ugly name.
|
||||
To be fair, this is perfectly fine if you're only using the generated DDL for testing.
|
||||
====
|
||||
|
||||
[[primary-key-column-mappings]]
|
||||
=== Mapping primary key joins between tables
|
||||
|
||||
The `@PrimaryKeyJoinColumn` is a special-purpose annotation for mapping:
|
||||
|
||||
- the primary key column of a `@SecondaryTable`—which is also a foreign key referencing the primary table, or
|
||||
- the primary key column of the primary table mapped by a subclass in a `JOINED` inheritance hierarchy—which is also a foreign key referencing the primary table mapped by the root entity.
|
||||
|
||||
|===
|
||||
| Annotation member | Purpose
|
||||
|
||||
| `name` | The name of the mapped foreign key column
|
||||
| `referencedColumnName` | The name of the column to which the mapped foreign key column refers
|
||||
| `columnDefinition` 💀| A DDL fragment that should be used to declare the column
|
||||
| `foreignKey` | A `@ForeignKey` annotation specifying the name of the `FOREIGN KEY` constraint
|
||||
|===
|
||||
|
||||
When mapping a subclass table primary key, we place the `@PrimaryKeyJoinColumn` annotation on the entity class:
|
||||
|
||||
[source,java]
|
||||
----
|
||||
@Entity
|
||||
@Table(name="People")
|
||||
@Inheritance(strategy=JOINED)
|
||||
class Person { ... }
|
||||
|
||||
@Entity
|
||||
@Table(name="Authors")
|
||||
@PrimaryKeyJoinColumn(name="personId") // the name of the primary key of the Authors table
|
||||
class Author { ... }
|
||||
----
|
||||
|
||||
But to map a secondary table primary key, the `@PrimaryKeyJoinColumn` annotation must occur inside the `@SecondaryTable` annotation:
|
||||
|
||||
[source,java]
|
||||
----
|
||||
@Entity
|
||||
@Table(name="Books")
|
||||
@SecondaryTable(name="Editions",
|
||||
pkJoinColumns = @PrimaryKeyJoinColumn(name="bookId")) // the name of the primary key of the Editions table
|
||||
class Book {
|
||||
@Id @GeneratedValue
|
||||
@Column(name="bookId") // the name of the primary key of the Books table
|
||||
Long id;
|
||||
|
||||
...
|
||||
}
|
||||
----
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue