title: Ordering by MethodSerializer
body: <p>I have a serializer that has a methodfield popular such as it returns a specific float. Is there a way to order by the serializermethodfield?</p>
  
  <p>Code Snippets:</p>
  
  <p><strong>serializers.py</strong></p>
  
  <pre><code>class PostListSerializer(serializers.HyperlinkedModelSerializer):
      popular = serializers.SerializerMethodField('popularity')
  
      def popularity(self, obj):
          time = datetime.datetime.now()
          n = obj.created.replace(tzinfo=None)
          r = time.replace(tzinfo=None)
          x = r - n
          a = float(str(x.total_seconds()//(60*60*24)))
          view = obj.post.count()
          return view / a
  </code></pre>
  
  <p><strong>views.py</strong></p>
  
  <pre><code>class PopularPostsSet(viewsets.ModelViewSet):
      queryset = Post.objects.all()
      serializer_class = PostListSerializer
      filter_backends = (filters.OrderingFilter,)
      ordering_fields = ('popular')
  </code></pre>