I have gotten the original concept (i.e., Planet is a Geopolitical Unit, Country is a Geopolitical Unit, Planet contains collection of Countries) to work. Two things that caused me problems in coming up with a solution empirically: - Having a subclass entity that is serializable of a base entity class that is not. Entities supposedly don't have to be serializable (Burke et al., Enterprise Java Beans 3.0, p. 52), but I have not tried going back and tried making them all non-serializable; - Not having an empty constructor at each level; I do not remember reading that this as a requirement of entity POJOs in Burke et al., but it is certainly not in the index!!! The code I wound up with does not look especially like my original posting, since I proceeded by getting one small piece working right, and then building on it incrementally, rather than trying to figure out what was wrong with what I had done originally. K/H View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3958323#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...