title: Django DateTimeField auto_now_add not working
body: <p>In one of the model i have set one timestamp field as follows:</p>
  
  <pre><code>created_datetime = models.DateTimeField(auto_now_add = True)
  </code></pre>
  
  <p>While in shell i am able to create a obj and save it, however in my application it is raising a exception that created_datetime field cannot be null.</p>
  
  <p>Confused where things went wrong!! How to reslove it.</p>