title: Access get_root of a MPTT model through a query
body: <p>Consider below example:</p>
  
  <pre><code>class ModelX(models.Model):
      fieldX = models.ForeignKey(ModelY)
  
  class ModelY(MPTTModel):
  
      def root(self):
          return get_root()
  
      root = property(root)
  </code></pre>
  
  <p>Now I would like to make query like this</p>
  
  <pre><code>ModelX.objects.filter(fieldX__root=match)
  </code></pre>
  
  <p>or better yet directly calling <code>get_root()</code> like this</p>
  
  <pre><code>ModelX.objects.filter(fieldX__get_root=match)
  </code></pre>
  
  <p>which would make the <code>root()</code> method superfluous.</p>
  
  <p>None of the above seem to work though. Why is that? </p>