title: Django: change the value of a field for all objects in a queryset
body: <p>I have a model <code>MyModel</code> with a boolean field <code>active</code></p>
  
  <p>Elsewhere, I am retrieving a queryset:</p>
  
  <pre><code>qs = MyModel.Objects.filter(....) 
  </code></pre>
  
  <p>how can I set <code>active=False</code> for all objects in this <code>qs</code>?</p>