code: def _check_relation(self, relation):
          
          selection = [val[0] for val in self.selection]
          if relation not in selection:
              raise ValueError(
                  ("The value '{value}' supplied doesn't match with the possible"
                   " values '{selection}' for the '{field_name}' field").format(
                       value=relation,
                       selection=selection,
                       field_name=self.name,
                   ))
          return relation
docs: Raise a `ValueError` if `relation` is not allowed among
          the possible values.