title: Django queryset update field increasing/descreasing its current value
body: <p>I am trying to change order of nodes in my tree. Everything works fine, but I would like to know if there is some beautiful, easy way of updating multiple fields by increasing its actual value by 1. Let me illustrate.</p>
  
  <pre><code>Objtree.objects.select_related().filter(pk__in = ids).update(sort_order = 1)
  </code></pre>
  
  <p>This code will change every sort_order column value to 1, but I would like to change it to something like:</p>
  
  <pre><code>Objtree.objects.select_related().filter(pk__in = ids).update(sort_order += 1)
  # or
  Objtree.objects.select_related().filter(pk__in = ids).update(self.sort_order = 1)
  </code></pre>
  
  <p>So... is there something like that? Nothing comes to my mind or my screen via googling.</p>
  
  <p>Thanks for halp!</p>