title: Django admin: sending signal on field change
body: <p>I need to trigger a signal if and only if form field "status" is updated. The signal works fine, but is being triggered regardless of any change submissions to the form.</p>
  
  <p>Below is my save_model override from admin.py, for the OrderAdmin class:</p>
  
  <pre><code>def save_model(self, request, obj, form, change):
      if not change:
          if not request.user.is_superuser:
              obj.organization = request.user
      if Order().is_dirty():
          custom_signals.notify_status.send(sender=self, status=obj.status)  
      obj.save()  
  </code></pre>
  
  <p>Here is my model:</p>
  
  <pre><code>class Order(DirtyFieldsMixin, models.Model):
  
  StatusOptions = (
    ('Pending Confirmation', 'Pending Confirmation'),
    ('Confirmed', 'Confirmed'),
    ('Modified', 'Modified'),
    ('Placed', 'Placed'),
    ('En Route', 'En Route'),
    ('Completed', 'Completed'),
    ('Cancelled', 'Cancelled'),
    )
  
  organization = models.ForeignKey(User, related_name='orders', default=1, help_text='Only visible to admins.')
  status = models.CharField(max_length=50, choices=StatusOptions, default=1, help_text='Only visible to admins.')
  order_name = models.CharField(max_length=22, blank=True, help_text='Optional. Name this order for easy reference (example: Munchies)')
  contact_person = models.ForeignKey(Contact, help_text='This person is in charge of the order. We may contact him/her regarding this order.')
  delivery_date = models.DateField('delivery day', help_text='Please use YYYY-MM-DD format (example: 2011-11-25)')
  </code></pre>