title: Manager isn't accessible via model instances
body: <p>I'm trying to get model objects instance in another one and I raise this error :</p>
  
  <pre><code> Manager isn't accessible via topic instance
  </code></pre>
  
  <p>Here's my model :</p>
  
  <pre><code>class forum(models.Model):
      # Some attributs
  
  class topic(models.Model):
      # Some attributs
  
  class post(models.Model):
      # Some attributs
  
      def delete(self):
          forum = self.topic.forum
          super(post, self).delete()
          forum.topic_count = topic.objects.filter(forum = forum).count()
  </code></pre>
  
  <p>Here's my view :</p>
  
  <pre><code>def test(request, post_id):
      post = topic.objects.get(id = int(topic_id))
      post.delete()
  </code></pre>
  
  <p>And I get :</p>
  
  <pre><code>post.delete()
  forum.topic_count = topic.objects.filter(forum = forum).count()
  Manager isn't accessible via topic instances
  </code></pre>