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.

def _reverse_call(self, related_method, *values):
        """
        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.
        """
        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)