I have NonUniqueObjectException thrown when upgrading my project from Hibernate 3 to Hibernate 4. I created minimal Hibernate 4 project for testing purposes, and managed to reproduce this exception by:

• having entity "Parent", and another entity "Child" that has a composite id that contains "Parent" entity
• and then in the same transaction fetching a "Parent" and then fetching "Child" by composite-id with:

(Child) session.get(Child.class, childParentPK)

When reverting this minimal project back to Hibernate 3 the exception is gone, and it all works fine. This could be a bug in Hibernate 4?

I have attached sample project, to run it and switch between Hibernate 3/4 consult readme.txt