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12. 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.

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