AppEngine datastore: “Object with id … is managed by a different Object Manager”

A persistent object can only be "managed" by one PersistenceManager. In DataNucleus this is backed internally by an "ObjectManager". The message says that you are trying to associate an object managed by one PM with a different PM. You can easily debug that by printing out the PM for each (persistent) object

JDOHelper.getPersistenceManager(obj);

Creating a PM is not expensive (creating a PMF is expensive).