Please help me this so I can avoid the big head ache... After a variety of debugging, this is the case: | ... | B b = a.getB(); | long id = saveOrUpdate(b); // Autoboxing. Originally an instance of Long | entityManager.flush(); | b.setId(id); // Autoboxing. Originally an instance of Long | ... | I discovered that the memory referance of b.getId() was put to null when I flushed the entity manager. Before the flushing I had done some logging, but it was not valid anymore. However; after flushing, persisting/merging A, and logging that b.getId() is still a valid value, I still get the same exception: | javax.ejb.EJBException: javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist: a.package.B | Is there some sort of configuration that I did not do? I thought that the merge method would check that the id of the entity was valid. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3973540#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...