title: Django self-recursive ManyToManyField filter query
body: <p>I have a model like so: </p>
  
  <pre><code>class Activity(models.Model):  
      title = models.CharField(max_length=200)
      summary = models.TextField(null=True, blank=True)  
      tasks = models.ManyToManyField('self', symmetrical=False, null=True, blank=True)  
  </code></pre>
  
  <p>It works like this, an activity can be linked to itself and the parent activity is called an 'Activity' and the child activity/activities will be called  'Task/Tasks'</p>
  
  <p>How do I filter the model to get all the 'Activities' and How do I filter the model to get all the 'Tasks' ? </p>
  
  <p>Thanks for all the help. </p>