关于 Simple References

By default all references are stored as a DBRef with the traditional $id, $db and $ref fields but if you want you can configure your references to be simple and only store a MongoId.


_id: ObjectId("5297115940e7b47c050041ae"),
created_at: ISODate("2013-11-28T09:48:09Z"),
user: {
$ref: "user",
$id: ObjectId("5295afb940e7b4f8000041a7"),
$db: "xxx"
}

如果用 simple references，就没有 $ref, $db信息了，这样存储空间更小，但会不会存在什么问题？