title: Django excluding specific instances from queryset without using field lookup
body: <p>I sometimes have the need to make sure some instances are excluded from a queryset.<br>
  This is the way I do it usually:  </p>
  
  <pre><code>unwanted_instance = MyModel.objects.get(pk=bad_luck_number)
  uninteresting_stuff_happens()
  my_results = MyModel.objects.exclude(id=unwanted_instance.id)
  </code></pre>
  
  <p>or, if I have more of them:</p>
  
  <pre><code>my_results = MyModel.objects.exclude(id_in=[uw_in1.id, uw_in2.id, uw_in3.id])
  </code></pre>
  
  <p>This 'feels' a bit clunky, so I tried:</p>
  
  <pre><code>my_ideally_obtained_results = MyModel.objects.exclude(unwanted_instance)
  </code></pre>
  
  <p>Which doesn't work. But I read <a href="https://stackoverflow.com/questions/1637897/django-queryset-excluding-entries-in-second-model/1638163#1638163">here on SO</a> that a subquery can be used as parameter for exclude.<br>
  Am I out of luck? Am I missing some functionality (checked the docs, but didn't find any useful pointer)</p>