title: Django Admin: Many-to-Many listbox doesn't show up with a through parameter
body: <p>I have the following models:</p>
  
  <pre><code>class Message(models.Model):
      date = models.DateTimeField()
      user = models.ForeignKey(User)    
      thread = models.ForeignKey('self', blank=True, null=True)
      ...
  
  class Forum(models.Model):
      name = models.CharField(max_length=24)
      messages = models.ManyToManyField(Message, through="Message_forum", blank=True, null=True)
      ...
  
  class Message_forum(models.Model):
      message = models.ForeignKey(Message)
      forum = models.ForeignKey(Forum)
      status = models.IntegerField()
      position = models.IntegerField(blank=True, null=True)
      tags = models.ManyToManyField(Tag, blank=True, null=True)
  </code></pre>
  
  <p>In the admin site, when I go to add/change a forum, I don't see the messages listbox as you'd expect. However, it shows up if I remove the 'through' parameter in the ManyToManyField declaration. What's up with that? I've registered all three models (plus Tag) to the admin site in admin.py.</p>
  
  <p>TIA</p>