code: def _validate(self, value):
          
          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
docs: 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.