title: django customize `default` order by
body: <p>I want to order queryset by sum of two fields.<br>
  Essentially, I want to change the default sort order of a model like this:(which doesn't work)</p>
  
  <pre><code>class Meta:
     ordering = ('-my_property_but_not_field', 'my_another_field')
  </code></pre>
  
  <p><code>my_property_but_not_field</code> will sum up two fields of the model. </p>
  
  <p>I've seen solutions which use:    </p>
  
  <p><code>extra</code> : <a href="https://stackoverflow.com/questions/3160798/django-order-by-sum-of-fields">Django order_by sum of fields</a><br>
  <code>annotate</code> : <a href="https://stackoverflow.com/questions/476017/django-queryset-order-by-method">Order a QuerySet by aggregate field value</a><br>
  <code>manager</code> : <a href="https://stackoverflow.com/questions/883575/custom-ordering-in-django">Custom ordering in Django</a></p>
  
  <p>With <code>extra</code> and <code>annotate</code>, I'll have to change every code which needs the new sorting order.  </p>
  
  <p>With <code>manager</code>, I don't have such a problem.<br>
  But I don't know how to emulate the order_by_1, order_by_2.  </p>