英文:
How to dereference dbref in mgo?
问题
以下是翻译好的内容：
var (type User struct{Id bson.ObjectId `bson:"_id"`Name string}type Post struct{Id bson.ObjectId `bson:"_id"`Uid stringUser Userref mgo.DBRefTitle string})> //尝试插入10000次id := bson.NewObjectId()user := &User{ id, "test"}db.C("users").insert(user)post := db.C("Post").insert(&Post{Uid: id.hex(), ref: mgo.DBRef{"ref":"users", "id": id}, Title:"test dbref"})>> //第一种方式很脏 -_-!//在mgo中如何实现MySQL的左连接（left join users on user.id=post.uid）？posts := new([]User)db.C("posts").Find(nil).All(posts)ids := []bson.ObjectIdfor _, p := range posts{ids = append(ids, p.Uid)}users := make([]User, len(ids))db.C("users").Find(bson.M{"_id": {"$in": ids}}).All(users)//然后设置User属性？for _,u := range users {for _, m := range m{if m.Uid == u.Id {m.User = m}}}> 第二种方式，使用ref属性，但是mgo.session会尝试查找idfor _,m := range posts{db.FindRef(m.ref).One(&m.User)}> //第三种方式，使用mapReduce？？这是我第一次使用golang + mongodb，那么实现dbref或者join的最佳方式是什么？谢谢
英文:
var (type User struct{Id bson.ObjectId `bson:"_id"`Name string}type Post struct{Id bson.ObjectId `bson:"_id"`Uid stringUser Userref mgo.DBRefTitle string})
> //try 10000 times inserts
id := bson.NewObjectId()user := &User{ id, "test"}db.C("users").insert(user)post := db.C("Post").insert(&Post{Uid: id.hex(), ref: mgo.DBRef{"ref":"users", "id": id}, Title:"test dbref"})
>> //first way so dirty -_-!
//mysql: left join users on user.id=post.uid, how to do in mgo ？
posts := new([]User)db.C("posts").Find(nil).All(posts)ids := []bson.ObjectIdfor _, p := range posts{ids = append(ids, p.Uid)}users := make([]User, len(ids))db.C("users").Find(bson.M{"_id": {"$in": ids}}).All(users)//and then set the User attribute?for _,u := range users {for _, m := range m{if m.Uid == u.Id {m.User = m}}}
> secondary way，with ref attribute, but mgo.session will try to findid
for _,m := range posts{db.FindRef(m.ref).One(&m.User)}
> //3th way, with mapReduce ??
it's my first golang + mongodb, so what is the best way to archive dbref or joins?
Thx
答案1
得分: 3
不使用DBRef，你可以使用手动引用的方法来连接两个或多个相关的文档。例如，你的结构可以如下所示：
type User struct{Id bson.ObjectId `bson:"_id"`Name string `json:"name"`}type Post struct{UserId bson.ObjectId `json:"userid"` // 对User的手动引用Title string}
然后，你可以使用$lookup聚合阶段来执行左外连接。例如，根据用户查找所有的帖子：
pipeline := []bson.M{bson.M{"$lookup": bson.M{"from": "posts","foreignField":"userid","localField":"_id","as":"posts",}},}result := []bson.M{}err := coll_users.Pipe(pipeline).All(&result)
示例结果：
{"_id": ObjectId("590ab726f4bab950360c2dbe"),"name": "test","posts": [{"_id": ObjectId("590ab72619663bad7743ff9e"),"userid": ObjectId("590ab726f4bab950360c2dbe"),"title": "test manual reference"}]}
除了将用户和帖子存储在单独的集合中，你还可以选择嵌入/子文档的方式。详见数据建模。
英文:
Instead of using DBRef, you can just use manual reference method for connecting two or more related documents. For example your struct can just look as below:
type User struct{Id bson.ObjectId `bson:"_id"`Name string `json:"name"`}type Post struct{UserId bson.ObjectId `json:"userid"` // manual ref to UserTitle string}
You can then use $lookup aggregation stage to perform a left outer join. For example, to find out all posts based on users:
pipeline := []bson.M{bson.M{"$lookup": bson.M{"from": "posts","foreignField":"userid","localField":"_id","as":"posts",},},}result := []bson.M{}err := coll_users.Pipe(pipeline).All(&result)
Example result:
{"_id": ObjectId("590ab726f4bab950360c2dbe"),"name": "test","posts": [{"_id": ObjectId("590ab72619663bad7743ff9e"),"userid": ObjectId("590ab726f4bab950360c2dbe"),"title": "test manual reference"}]}
Alternative to storing users and posts in separate collections, you could also embed/sub-document. See also Data Modelling
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