title: How can we get a model instance not by model pk in DRF
body: <p>I would like to get a model instance from my record in <code>django-rest-framework (DRF)</code> api.</p>
  
  <p>On default we can get an instance of model by a particular <code>pk</code> </p>
  
  <pre><code>/api/animal/1/
  </code></pre>
  
  <p>But how can we get an instance with another model property supposed the <code>animal</code> model may have a <code>herd_id</code> property?</p>
  
  <p>I am using this viewset to handle the crud.</p>
  
  <pre><code>class AnimalViewSet(viewsets.ModelViewSet):
      """
      This viewset automatically provides `list`, `create`, `retrieve`,
      `update` and `destroy` actions.
      """
      queryset = Animal.objects.all()
      serializer_class = AnimalSerializer
  </code></pre>