title: Django filtering QueryString by MultiSelectField values?
body: <p>I'm using <a href="https://github.com/goinnn/django-multiselectfield" rel="nofollow">MultiSelectField</a> to store a topic/topic's of conversation</p>
  
  <p>My model looks somewhat like this:</p>
  
  <pre><code>class Conversation(models.Model):
      (...)
      note= models.CharField(max_lenght=250)
      TOPIC_CHOICES =(
          (1,"about cats"),
          (2, "about dogs"),
          (3, "about fluffy little things"),
      )
      topic =MultiSelectField(choices=TOPIC_CHOICES)
  </code></pre>
  
  <p>I'm using <code>ListView</code> and filtering by GET parameters inside <code>get_queryset</code>:</p>
  
  <p>form extract:</p>
  
  <pre><code>class SearchForm(Form):
      (...)
      topic = MultipleChoiceField(choices=Conversation.TOPIC_CHOICES, required=False)
  </code></pre>
  
  <p>get_queryset extract :</p>
  
  <pre><code>(...)
  if form.cleaned_data['topic']:
                  search_params.update({'topic__in': form.cleaned_data['topic']})
  (...)
  return qs.filter(**search_params)
  </code></pre>
  
  <p>This method worked fine for single value choice fields. </p>
  
  <p>But in this case if I for ex. select in form "about cats" I got only objects that topic is set to cats <strong>only</strong>("about cats" and nothing else -single value). </p>
  
  <p>What I want is all objects in which one of topic values is 1-"about cats". This mean if some object have topic=<code>1,3</code>(cats and fluffy things) it should appear too</p>
  
  <p>Second scenario: I select "about cats" and "about dogs" in form - I want <strong>all</strong> objects that have cats as one of the topic's <strong>and all</strong> objects that have a dogs as one of the topic's
  Right now when I select more than one option for ex. cats and dogs I get all that have <strong>only</strong> cats and all that got <strong>only</strong> dogs as a topic</p>
  
  <p>Is there any other field lookup string instead of <code>__in</code> that will achieve that?
  If not what is most hassle free way to do that?</p>