private function getDbRefValue($url)
    {
        if ($url === null) {
            return null;
        }

        try {
            $extref = $this->converter->getExtReference($url);
            return \MongoDBRef::create($extref->getRef(), $extref->getId());
        } catch (\InvalidArgumentException $e) {
            //make up some invalid refs to ensure we find nothing if an invalid url was given
            return \MongoDBRef::create(false, false);
        }
    }