title: DRF: how to call a serializer (as a nested serializer field) before it is defined?
body: <pre><code>class ASerializer(serializers.ModelSerializer):
    b = Bserializer()
    ...
  
  class BSerializer(serializers.ModelSerializer):
    ...
  </code></pre>
  <p>I need to call BSerializer before it is defined. How do I do that?</p>