          Returns a default instance of the pk field.
          """
          return self.get_field(model_field)
  
      def get_nested_field(self, model_field):
          """
          Creates a default instance of a nested relational field.
          """
          class NestedModelSerializer(ModelSerializer):
              class Meta:
 
</s> remove         return None
 </s> add         if self.opts.fields and model_field.name in self.opts.fields:
            return self.get_field(model_field) </s> remove                 model = model_field.rel.to
 </s> add                 model = model_class </s> remove     def get_related_field(self, model_field, to_many=False):
 </s> add     def get_related_field(self, model_field, rel=None, to_many=False):
    def get_nested_field(self, model_field, rel=None):
          Returns a default instance of the pk field.
          """
          return self.get_field(model_field)
  
    def get_nested_field(self, model_field, rel=None):
          """
          Creates a default instance of a nested relational field.
          """
          class NestedModelSerializer(ModelSerializer):
              class Meta:
keep keep keep keep replace keep keep keep keep keep
 First pass on automatically including reverse relationship 
rest_framework/serializers.py