title: Pass context from one serializer to another?
body: <p>With Django REST Framework, I have 2 serializers: <code>PageSerializer</code> and <code>CommentSerializer</code>.</p>
  
  <p><code>CommentSerializer</code> depends on some extra "context" value, but it doesn't get it directly, instead it needs to get it from <code>PageSerializer</code>, since they have a nested relationship.</p>
  
  <p>So I need to have something like this:</p>
  
  <pre><code>class CommentSerializer(serializers.ModelSerializer):
      ...
      my_field = serializers.SerializerMethodField()
  
      def get_my_field(self, comment):
          my_value = self.context['my_value']
          ...
  
  class PageSerializer(serializers.ModelSerializer):
      ...
      comments = CommentSerializer(
          many=True,
          context={'my_value': my_value} # my_value doesn't exist until __init__ is called, so I can't pass it
      )
  
  ...
  my_value = 1
  page_serializer = PageSerializer(page, context={'my_value': my_value})
  </code></pre>
  
  <p>But, of course, this code can't work.<br>
  What kind of workaround can I do here?</p>