How to dereference dbref in mgo? - go

Question

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
}
)
//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.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
}
}
}
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

Answer 1

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 User
Title 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