title: Update dynamic field django queryset
body: <p>i want build update  django <code>queryset</code> with dynamic field like param.
  Example my field is <code>planned</code>:</p>
  
  <pre><code> def update_data(period, old_planned, new_planned):
      Cost.objects.filter(period=period).update(
              planned=F('planned') - old_planned + new_planned)
  </code></pre>
  
  <p>If i want pass <code>planned</code> field like param input in <code>def</code> like <code>def(period, old_planned, new_planned, update_field)</code>, how to do it with keep use <code>F</code> inside ?</p>