code: def _validate(self, value):
          
          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))
docs: Validate that a given new pk to set is always set to None, then return
          a new pk