Extend Google AppEngine User in JRuby?

Question

I'm working with JRuby and DataMapper running on Google AppEngine.

I want to add a property to the AppEngine::User like :active_calendar which is a reference to a Calendar kind. I was able to do something in Python this way using a back reference. Are these possible in JRuby?

Is this possible? Do I need to subclass the User? Can I even do that? If so - how?

Thanks!