<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def find_or_fail(self, id, columns=None):
 """ Find a model by its primary key or raise an exception :param id: The primary key value :type id: mixed :param columns: The columns to retrieve :type columns: list :return: The found model :rtype: orator.Model :raises: ModelNotFound """

        result = self.find(id, columns)

        if isinstance(id, list):
            if len(result) == len(set(id)):
                return result
        elif result:
            return result

        raise ModelNotFound(self._model.__class__)