The majority of the many-to-many associations and collections of values shown previously all map to tables with composite keys, even though it has been have suggested that entities should have synthetic identifiers (surrogate keys). A pure association table does not seem to benefit much from a surrogate key, although a collection of composite values might. It is for this reason that Hibernate provides a feature that allows you to map many-to-many associations and collections of values to a table with a surrogate key.
<idbag> element lets you map a
Collection) with bag semantics. For example:
<idbag name="lovers" table="LOVERS">
<collection-id column="ID" type="long">
<many-to-many column="PERSON2" class="Person" fetch="join"/>
<idbag> has a synthetic id generator, just like an entity class. A different surrogate key is assigned to each collection row. Hibernate does not, however, provide any mechanism for discovering the surrogate key value of a particular row.
The update performance of an
<idbag> supersedes a regular
<bag>. Hibernate can locate individual rows efficiently and update or delete them individually, similar to a list, map or set.
In the current implementation, the
native identifier generation strategy is not supported for
<idbag> collection identifiers.