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

		foreach ($this->_columns as $name => $fields) {
			if ($name === '') {
				$flip = array_flip($fields);

				$keys = array_flip($model::meta('key'));
				$keys = array_intersect_key($flip, $keys);

				foreach ($keys as &$key) {
					$key += $index;
				}
				return array_flip($keys);
			}
			$index += count($fields);
		}
		return [];
	}