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6. Top-level Collections


Definition:
A top-level collection is a collection defined outside of the scope of an individual class and available for use in all classes in the mapping files.

Scenario:
We have one first-rank class, Foo, and a collection of Strings (e.g. people's names) which we wish to make available to other classes without constantly declaring set definitions inside each one.

Set Foo.getNames() // returns a collection of String instances

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

<set role="names" table="names">
     <key column="id" type="string">
     <generator class="uuid.hex"/> </key>
     <element column="name" type="string"/>
 </set>
<class name="Foo" table="foo">
     ...
     <collection name="names" column="name_id" role="names"/>
 </class>

Note that a top-level collection needs its own key generator, and that this cannot be of the assigned type since it is never exposed to a calling application.

Table Schema:

Foo

id

person_id

Person

id

name


Again, Person does not represents a first-rank class. It is simply a collection of second-rank persistent objects - in this case Strings. Note also that Names simply has id not foo_id. This is to allow it to be used by a variety of first-rank classes. Also, because we cannot use Person's id as a foo_id, we have added a person_id FK to Foo.

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

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