title: Queryset API distinct() does not work?
body: <pre><code>class Message(models.Model):
      subject = models.CharField(max_length=100)
      pub_date = models.DateTimeField(default=datetime.now())
  
  class Topic(models.Model):
      title = models.CharField(max_length=100)
      message = models.ManyToManyField(Message, verbose_name='Discussion') 
  </code></pre>
  
  <p>I want to get order all the topics according to the latest message object attached to that topic.
  I executed this query but this does not give the distinct queryset.</p>
  
  <pre><code>&gt;&gt; Topic.objects.order_by('-message__pub_date').distinct()
  </code></pre>