13. Collections of Components

Definition: Collections of components are also possible. These can be especially useful when dealing with a complex top-level collection.

Scenario: We have one first-rank class, Foo, and a top-level collection of second-rank class FooSecond which Foo holds.

Set Foo.getSeconds() // returns a collection of FooSecond instances

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

<set role="seconds">      <key column="id" type="string">           <generator class="uuid.hex"/>      </key>      <composite-element class="FooSecond">           <property name="firstName"/>           <property name="lastName"/>      </composite-element> </set> <class name="Foo" table="foo"> ...      <collection name="seconds" column="seconds_id" role="seconds"/> </class>

Table Schema:

Foo
id	seconds_id

Seconds
id	firstName	lastName

This time we have two tables. Foo has a seconds_id FK column and Seconds appears as a collection table in its own right. The table structure is the same as for a standard top-level collection, but this time Seconds is being treated as a component object - a composite of two columns in the collection table - and set as a single property of Foo.

Bidirectionality:
There's no bidirectional relationship available here as there is only one first-rank class involved.