                      ret[field_name] = self.get_related_field(*relation_info)
  
          return ret
  
      def get_pk_field(self, model_field):
          """
          Returns a default instance of the pk field.
          """
 
</s> remove     Any fields included as attributes on either the class or it's superclasses
    will be include in the `base_fields` dictionary.
 </s> add     Any instances of `Field` included as attributes on either the class
    or on any of its superclasses will be include in the
    `base_fields` dictionary. </s> remove     def get_default_fields(self):
        fields = super(HyperlinkedModelSerializer, self).get_default_fields()

        if self.opts.view_name is None:
            self.opts.view_name = self.get_default_view_name(self.opts.model)

        url_field_name = api_settings.URL_FIELD_NAME
        if url_field_name not in fields:
            ret = fields.__class__()
            ret[url_field_name] = self.get_url_field()
            ret.update(fields)
            fields = ret

        return fields

    def get_pk_field(self, model_field):
        if self.opts.fields and model_field.name in self.opts.fields:
            return self.get_field(model_field)

 </s> add 
    def get_url_field(self):
        return None

                      ret[field_name] = self.get_related_field(*relation_info)
  
          return ret
  
    def get_url_field(self):
        return None

      def get_pk_field(self, model_field):
          """
          Returns a default instance of the pk field.
          """
keep keep keep add keep keep keep keep
 Added help_text argument to fields 
rest_framework/serializers.py