public static function getOne(Item $parent, Item $child, $options = [])
    {
        $key = md5($parent->systemId . "." . $child->systemId);
        if (isset(self::$_relationships[$key])) {
            self::$_relationships[$key]->mergeOptions($options);
        } else {
            self::$_relationships[$key] = new Relationship($parent, $child, $options);
        }

        return self::$_relationships[$key];
    }