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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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))