protected function _keys($keys) {
		$config = $this->_config;

		if (!($related = ($config['type'] == 'belongsTo') ? $config['to'] : $config['from'])) {
			return array();
		}
		if (class_exists($related)) {
			return array_combine((array) $keys, (array) $related::key());
		}
		throw new ClassNotFoundException("Related model class '{$related}' not found.");
	}