code: def _reverse_call(self, related_method, *values):
          
          related_fields = self._to_fields(*values)
          for related_field in related_fields:
              if callable(related_method):
                  related_method(related_field, self.instance._pk)
              else:
                  getattr(related_field, related_method)(self.instance._pk)
docs: Convert each value to a related field, then call the method on each
          field, passing self.instance as argument.
          If related_method is a string, it will be the method of the related field.
          If it's a callable, it's a function which accept the related field and
          self.instance.