title: Using another model manager's code in a Django custom manager class
body: <p>I have two models, say, <code>Question</code> and <code>Topic</code>. </p>
  
  <p>I am trying to add methods to the Question model's custom manager, e.g. some method that filters by <code>Topic</code>.</p>
  
  <p>I can't seem to use the other manager's code for this (cannot <code>import Topic</code> either, so I can't do <code>Topic.objects...</code>)</p>
  
  <p>In <code>class QuestionManager</code></p>
  
  <pre><code>def my_feed(self, user):
         topics = TopicManager().filter(user=user) # 1st approach
         #topics = Topic.objects.filter(user=user) # 2nd line
         # do something with topics
  </code></pre>
  
  <p>class TopicManager
       ....</p>
  
  <p>Using 1st approach, I get the following error:</p>
  
  <pre><code>virtualenv/local/lib/python2.7/site-packages/django/db/models/sql/query.pyc in get_meta(self)
      219         by subclasses.
      220         """
  --&gt; 221         return self.model._meta
      222 
      223     def clone(self, klass=None, memo=None, **kwargs):
  
  AttributeError: 'NoneType' object has no attribute '_meta'
  </code></pre>
  
  <p>I can't use the 2nd line since I can't import Topic, since Topic depends on the TopicManager in this file. Is there a workaround for this?</p>