Looks like the problem is due to a combination of two factors: *Class Inner has two parents, Outer and Container
AND *Class Container mantains a list of Inners. Changing to @PersistenceCapable class Container{ @PrimaryKey @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) Key key; @Persistent ArrayList<Outer> outers; @Persistent Inner inners; public void addOuters( int n){ outers = new ArrayList<Outer>(); for (int i = 0; i < n; i++){ outers.add(new Outer()); } } public void addInners(int n){ for (int i = 0; i < n; i++){ inner = new Inner(); } } } Makes everything work fine. I have a very similar model which works well in production (it's been up for nearly one month now). Hope this helps people facing similar issues. Maybe a short note in documentation could help. Regards Lorenzo -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to google-appengine-j...@googlegroups.com. To unsubscribe from this group, send email to google-appengine-java+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en.