title: use get_object_or_404() method with mongoengine
body: <p>I'm starting to learn Django using Mongodb, creating a poll app following the djangoproject tutorial. I have a problem at the moment of raising a 404 error.
  If I do this:</p>
  
  <pre><code>def detail(request, poll_id):
   try:
      poll = Poll.objects.get(pk=poll_id)
   except Poll.DoesNotExist:
      raise Http404
   return render(request, 'polls/detail.html', {'poll': poll})
  </code></pre>
  
  <p>it works, but if I try to use a django shortcut:</p>
  
  <pre><code>def detail(request, poll_id):
    poll = get_object_or_404(Poll, pk=poll_id)
    return render(request, 'polls/detail.html', {'poll': poll})
  </code></pre>
  
  <p>I get this error</p>
  
  <pre><code>Object is of type 'Poll', but must be a Django Model, Manager, or QuerySet
  </code></pre>
  
  <p>I think this error occurs because in models.py I've defined the Poll model as a mongoengine Document and not as a django Model</p>
  
  <pre><code>class Poll(Document):
  </code></pre>
  
  <p>What should I do to have the get_object_or_404() method working?</p>