var (
    type User struct{
        Id bson.ObjectId `bson:"_id"`
        Name string
    }

type Post struct{
    Id bson.ObjectId `bson:"_id"`
    Uid string
    User User
    ref mgo.DBRef
    Title string
    }
)


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"})



//mysql:在user.id = post.uid上左加入用户,在mgo中怎么做?
posts := new([]User)
db.C("posts").Find(nil).All(posts)

ids := []bson.ObjectId
for _, 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
       }
   }
}


for _,m := range posts{
   db.FindRef(m.ref).One(&m.User)
}



这是我的第一个golang + mongodb,那么归档dbref或join的最佳方法是什么?

谢谢

最佳答案

除了使用DBRef之外,您还可以使用manual reference方法来连接两个或更多相关文档。例如,您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"` // manual ref to User
    Title string
}

然后,您可以使用$lookup aggregation stage执行左外部联接。例如,要查找基于用户的所有帖子,请执行以下操作:
pipeline := []bson.M{
        bson.M{"$lookup": bson.M{
                            "from": "posts",
                            "foreignField":"userid",
                            "localField":"_id",
                            "as":"posts",
                            },
              },
        }

result := []bson.M{}
err := coll_users.Pipe(pipeline).All(&result)

结果示例:
{
  "_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之外,您还可以嵌入/子文档。另请参阅Data Modelling

09-28 06:29