title: Filtering models with ReferenceProperties
body: <p>I'm using google app engine, and am having trouble writing querys to filter ReferenceProperties.</p>
  
  <p>eg.</p>
  
  <pre><code>class Group(db.Model):
      name = db.StringProperty(required=True)
      creator = db.ReferenceProperty(User)
  
  class GroupMember(db.Model):
      group = db.ReferenceProperty(Group)
      user = db.ReferenceProperty(User)
  </code></pre>
  
  <p>And I have tried writing something like this:</p>
  
  <pre><code>members = models.GroupMember.all().filter('group.name =', group_name)
  </code></pre>
  
  <p>and various other things that don't work. Hopefully someone can give me a prod in the right direction...</p>