title: How do I query the length of a Django ArrayField?
body: <p>I have an ArrayField in a model, I'm trying to annotate the length of this field ( so far without any luck) </p>
  
  <p><code>F('field_name__len')</code> won't work since join is not allowed inside <code>F()</code>.  Even 
  <code>ModelName.objets.values('field_name__len')</code> is not working</p>
  
  <p>Any idea?</p>
  
  <p>I'm using <strong>django 1.11</strong></p>