title: Django's DateTimeField's "auto_now" doesn't seem to work
body: <p>I have a <code>Comment</code> model. It has following timestamp fields:</p>
  
  <pre><code>created = models.DateTimeField(auto_now_add=True, blank=True, null=True)
  last_edit = models.DateTimeField(auto_now=True, blank=True, null=True)
  </code></pre>
  
  <p>Now when I update a comment using this format: <code>Comment.objects.filter(...).update(text="some new text")</code>, the <code>last_edit</code> field doesn't get updated while the text of the comment does get updated. What's the issue?</p>
  
  <p>Update: Also, I am using <code>filter</code> because <code>update</code> doesn't work with <code>get</code>, i.e.,  <code>Comment.objects.get(...).update(...)</code> won't work. What I actually want to do is <code>get</code> because I am sure only one comment has to updated at one time.</p>