This is another one of the common secret pitfalls of ORM frameworks. I first got burned in Kodo, but I think since then also in Hibernate and/or OpenJPA. I think it deserves note in the OpenJPA documentation in a section that discusses the cases where an ORM model doesn't exactly match its persistent counterpart.
http://saloon.javaranch.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic&f=78&t=004041&p=1
