protected function isKeyAllowed($key)
    {
        /*
         * Let the core columns through
         */
        if ($key == 'id' || $key == 'value' || $key == 'item') {
            return true;
        }

        /*
         * Let relations through
         */
        if ($this->model->hasRelation($key)) {
            return true;
        }

        return false;
    }