Validate that a given new pk to set is always set, and return it.
        The returned value should be normalized, and will be used without check.

def _validate(self, value):
        """
        Validate that a given new pk to set is always set, and return it.
        The returned value should be normalized, and will be used without check.
        """
        if value is None:
            raise ValueError('The pk for %s is not "auto-increment", you must fill it' %
                            self._model.__name__)
        value = self.normalize(value)

        # Check that this pk does not already exist
        if self.exists(value):
            raise UniquenessError('PKField %s already exists for model %s)' %
                                  (value, self._instance.__class__))

        return value