title: Django pre_save not saving changes
body: <p>I need to modify some value before saving model:</p>
  
  <pre><code>@receiver( pre_save )
  def model_pre_save( sender, instance, *args, **kwargs ):
      print instance.path # prints "old_path"
      instance.path = some_path()
      print instance.path # prints "new_path"
  </code></pre>
  
  <p>But after save <code>instance.path</code> still equals <code>"old_path"</code>.</p>