title: GAE: Child object that is not in the class
body: <p>I am trying to duplicate the pattern described in <a href="http://code.google.com/events/io/2009/sessions/BuildingScalableComplexApps.html" rel="nofollow">Building Scalable, Complex Apps on App Engine</a>.  I am having trouble understanding how to tell GAE that Message index is a child of Message.  From <a href="http://code.google.com/appengine/docs/java/datastore/dataclasses.html#Child_Objects_and_Relationships" rel="nofollow">google</a>, they say that you create a child object by including it in the parent class, but this is exactly what we are trying to prevent.  So how do I store an entity as a child without including it in the parent class.  </p>