title: Check previous model value on save
body: <p>I have a model that saves an Excursion. The user can change this excursion, but I need to know what the excursion was before he change it, because I keep track of how many "bookings" are made per excursion, and if you change your excursion, I need to remove one booking from the previous excursion. </p>
  
  <p>Im not entirely sure how this should be done.</p>
  
  <p>Im guessing you use a signal for this?</p>
  
  <p>Should I use pre_save, pre_init or what would be the best for this? </p>
  
  <p>pre_save is not the correct one it seems, as it prints the new values, not the "old value" as I expected</p>
  
  <pre><code>@receiver(pre_save, sender=Delegate)
  def my_callback(sender, instance, *args, **kwargs):
      print instance.excursion
  </code></pre>