title: Django ManyToMany filter()
body: <p>I have a model:</p>
  
  <pre><code>class Zone(models.Model):
      name = models.CharField(max_length=128)
      users = models.ManyToManyField(User, related_name='zones', null=True, blank=True)
  </code></pre>
  
  <p>And I need to contruct a filter along the lines of:</p>
  
  <pre><code>u = User.objects.filter(...zones contains a particular zone...)
  </code></pre>
  
  <p>It has to be a filter on User and it has to be a single filter parameter. The reason for this is that I am constructing a URL querystring to filter the admin user changelist: <code>http://myserver/admin/auth/user/?zones=3</code></p>
  
  <p>It seems like it should be simple but my brain isn't cooperating!</p>