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13. Composite Id


Definition:
Composite Identifiers are PK identifiers for first-rank classes that consist of more than one column.

Scenario:
Foo has a primary key that is of type Person. Person is made up of a String name and an Address type.

Person Foo.getId()   // PK is multi-column mapped 

Hibernate Mapping:
In Hibernate, this could be mapped as follows:

<class name="Foo" table="foo">
     <composite-id name="id" class="Person">
         <key-property name="name" type="string" column="name"/>
         <key-many-to-one name="address" class="Address" column="addr_id"/>
     </composite-id>
     <property name="age" column="age" type="string"/>
 </class>
<class name="Address" table="address">
     ...
 </class>

Foo has a composite id of type Person. This is mapped to two columns: 1) Name, 2) a many-to-one relationship with the Address class.

Either <key-property> or <key-many-to-one> declarations may exist within a composite id.

Table Schema:

Foo

name

addr_id

age

Address

id


So Foo has two PK-columns: addr_id and name. These are combined to initialise Person instances.

Bidirectionality:
Bidirectionality has no meaning here.

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