A bag is an unordered persistent collection that permits duplicate element values and ignores element indexes. In Java, map it with Collection and commonly initialize it with ArrayList. Hibernate stores the elements without maintaining list positions or enforcing set-style uniqueness. section 8.1.5
Choose a bag when the collection semantics and database design support these conditions:
- Use a surrogate collection-table key when duplicate values must be distinguishable; Hibernate’s proprietary @CollectionId supplies this identifier. section 8.1.5 section 8.2.3
- Prefer a bag over a List when order is irrelevant, and over a Set when duplicate values are valid. section 8.1.5 section 9.2.1
- For a bidirectional one-to-many association, a bag can add a new entity without loading the existing collection, improving behavior for potentially large collections. section 9.2.1
- Avoid eager-fetching two bag collections simultaneously because separate collection queries can produce an undesirable Cartesian product. section 9.2.1
Bag of entity references
A bidirectional one-to-many mapping can declare the collection as Collection<Bid> while retaining the association owner on Bid: @OneToMany(mappedBy = "item") private Collection<Bid> bids = new ArrayList<>(); Adding a new Bid can persist it without loading every existing bid, provided the collection is not iterated.
