Ordering The Set Of Associated Entities In @ManyToMany Association Via @OrderBy

Description: In case of @ManyToMany association, we always should rely on Set (not on List) for mapping the collection of associated entities (entities of the other parent-side). Why? Well, please see Prefer Set Instead of List in @ManyToMany Relationships. But, is well-known that HashSet doesn't have a predefined entry order of elements. If this is an issue then this application relies on @OrderBy which adds an ORDER BY clause in the SQL statement. The database will handle the ordering. Further, Hibernate will preserve the order via a LinkedHashSet.

This application uses two entities, Author and Book, involved in a lazy bidirectional many-to-many relationship. First, we fetch a Book by title. Further, we call getAuthors() to fetch the authors of this book. The fetched authors are ordered descending by name. The ordering is done by the database as a result of adding @OrderBy("name DESC"), and is preserved by Hibernate.

Key points:

• ask the database to handle ordering and Hibernate to preserve this order via @OrderBy
• this works with HashSet, but doesn't provide consistency across all transition states (e.g., transient state)
• for consistency across the transient state as well, consider using explicitly LinkedHashSet instead of HashSet

Note: Alternatively, we can use @OrderColumn. This gets materialized in an additional column in the junction table. This is needed for maintaining a permanent ordering of the related data.

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