title: Using Django custom manager function on an already filtered queryset
body: <p>Consider the following case:</p>
  
  <pre><code>class MyModelManager(models.Manager):
      def my_filter(self):
       return [some code here].filter(field_A__gt=3)
  
  class MyModel(models.Model):
      # some fields
      objects = MyModelManager()
  
  
  # The way I'd like to use it:
  qs = MyModel.objects.filter(field_B__lt=1)
  # some code
  qs = qs.my_filter()
  </code></pre>
  
  <p>Notice that I'd like to use the custom <code>my_filter()</code> function on the already filtered queryset. What code should I place in the <code>[some code here]</code> above? Is the structure as a whole correct?</p>