protected static function modelEntityType($model)
    {
        if ($model instanceof QuestionInterface) {
            return self::ENTITY_QUESTION;
        } elseif ($model instanceof AnswerInterface) {
            return self::ENTITY_ANSWER;
        } else {
            $className = get_class($model);
            throw new UnknownClassException("Model class '{$className}' not supported");
        }
    }