      def build_url_field(self, field_name, model_class):
          """
          Create a field representing the object's own URL.
          """
          field_class = HyperlinkedIdentityField
          field_kwargs = get_url_kwargs(model_class)
  
          return field_class, field_kwargs
  
      def build_unknown_field(self, field_name, model_class):
 
</s> remove         field_class = self.serializer_related_class
 </s> add         field_class = self.serializer_related_field </s> remove     serializer_related_class = PrimaryKeyRelatedField
 </s> add     serializer_related_field = PrimaryKeyRelatedField
    serializer_url_field = HyperlinkedIdentityField
    serializer_choice_field = ChoiceField </s> remove     serializer_related_class = HyperlinkedRelatedField
 </s> add     serializer_related_field = HyperlinkedRelatedField </s> remove             field_class = ChoiceField
 </s> add             field_class = self.serializer_choice_field
        field_class = self.serializer_url_field
      def build_url_field(self, field_name, model_class):
          """
          Create a field representing the object's own URL.
          """
        field_class = self.serializer_url_field
          field_kwargs = get_url_kwargs(model_class)
  
          return field_class, field_kwargs
  
      def build_unknown_field(self, field_name, model_class):
keep keep keep keep replace keep keep keep keep keep
 Minor cleanups/improvements to ModelSerializer API 
rest_framework/serializers.py