title: Use field value in limit_choices_to in Django
body: <p>I have two models <code>Project</code> and <code>Group</code>. My groups belong to a specific project. My groups have the fields <code>project = ForeignKey(Project)</code> and <code>parent = ForeignKey('self')</code>.</p>
  
  <p>Can I use <code>limit_choices_to</code> to make sure the options in foreign key <code>parent</code> only consist of groups inside the same project?</p>
  
  <p>I'm thinking of something like</p>
  
  <pre><code>def limit_choices_to(self):
      return {'project': self.project}
  </code></pre>