Validate that a given new pk to set is always set to None, then return
        a new pk

def _validate(self, value):
        """
        Validate that a given new pk to set is always set to None, then return
        a new pk
        """
        if value is not None:
            raise ValueError('The pk for %s is "auto-increment", you must not fill it' %
                            self._model.__name__)
        key = self._instance.make_key(self._model._name, 'max_pk')
        return self.normalize(self.connection.incr(key))