Hibernate's <idbag> facility allows you to map many-to-many associations and collections of values to a single table with a surrogate key
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.
The element lets you map a List (or Collection) with bag semantics. For example:
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 supersedes a regular . Hibernate can locate individual rows efficiently and update or delete them individually, similar to a list, map or set.