title: Django Filter if field is not None
body: <p>I have a model where I can set <code>expire_date</code>.
  I want to filter the model and when there's a expire_date set it should filter if the expire_date is lower than now.</p>
  
  <p>I tried it to do with Case and When but I can't filter in a When function, can I?</p>
  
  <pre><code>News.objects.all().annotate(is_expired=Case(When(F("expire_date") != None, then=False if F("expire_date") &lt;= datetime.now() else True))).filter(is_expired=False)
  </code></pre>