title: Adding client side calculated fields to django admin
body: <p>Say I've got a model like so:</p>
  
  <pre><code>class Spam(models.Model):
      a = models.IntegerField()
      b = models.IntegerField()
  </code></pre>
  
  <p>On the admin create/edit form. I want to add a non-editable field "c", that will contain the sum of whatever has been entered in a and b.</p>
  
  <p>I have a boatload of ideas on how to accomplish this, and none of them sound very good.</p>
  
  <p>Can anyone point me in the right direction?</p>