Fetching Associations In Batches Via @BatchSize

Description: This application uses Hibernate specific @BatchSize at class/entity-level and collection-level. Consider Author and Book entities invovled in a bidirectional-lazy @OneToMany association.

• First use case fetches all Author entities via a SELECT query. Further, calling the getBooks() method of the first Author entity will trigger another SELECT query that initializes the collections of the first three Author entities returned by the previous SELECT query. This is the effect of @BatchSize at Author's collection-level.

• Second use case fetches all Book entities via a SELECT query. Further, calling the getAuthor() method of the first Book entity will trigger another SELECT query that initializes the authors of the first three Book entities returned by the previous SELECT query. This is the effect of @BatchSize at Author class-level.

Note: Fetching associated collections in the same query with their parent can be done via JOIN FETCH or entity graphs as well. Fetching children with their parents in the same query can be done via JOIN FETCH, entity graphs and JOIN as well.

Key points:

• Author and Book are in a lazy relationship (e.g., @OneToMany bidirectional relationship)
• Author entity is annotated with @BatchSize(size = 3)
• Author's collection is annotated with @BatchSize(size = 3)

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