[Originally shared on G+ the 14th of August 2012] If you are interested in EclipseLink support for extensible entities, besides the official documentation you may want to have a look at this forum thread:http://www.eclipse.org/forums/index.php/mv/msg/357791/880331/#msg_880331The last post is under my name but credits go to my colleague Paolo Di Patria for writing a working test case.