21.5. Conclusion

There is quite a bit to digest here and it might look confusing first time around. However, in practice, it all works out very nicely. Most Hibernate applications use the parent / child pattern in many places.
We mentioned an alternative in the first paragraph. None of the above issues exist in the case of <composite-element> mappings, which have exactly the semantics of a parent / child relationship. Unfortunately, there are two big limitations to composite element classes: composite elements may not own collections, and they should not be the child of any entity other than the unique parent.