protected function _keyIndex() {
		if (!($model = $this->_model) || !isset($this->_columns[''])) {
			return [];
		}
		$index = 0;

		foreach ($this->_columns as $name => $fields) {
			if ($name === '') {
				if (($offset = array_search($model::meta('key'), $fields)) === false) {
					return [];
				}
				return [$index + $offset => $model::meta('key')];
			}
			$index += count($fields);
		}
		return [];
	}