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11. Components


Definition:
Components are Java classes that are populated from selected columns of a parent class' table. This allows second-rank classes to exist within a first-rank class whilst still mapping to a single table for efficiency.

Scenario:
We have one first-rank class, Foo, and a second-rank class, FooSecond.

FooSecond Foo.getSecond() // returns enclosed second-rank instance

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

<class name="Foo" table="foo">
      ...
      <component name="second" class="FooSecond">
           <property name="firstName"/>
           <property name="lastName"/>
      </component>
 </class>

Table Schema:

Foo

id

firstName

lastName


Here, Foo is mapped using id from table Foo. FooSecond is mapped from the same table using firstName and lastName.

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

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