Why is my Go code using mgo failing to find a MongoDB document by its _id, even though the _id exists?

Find by id with mgo

Question: I would like to find a data by _id. I know that this data exists and that this _id exist (I've tested it with pymongo). However, the code below doesn't find it:

type id_cookie struct {
    IdCookie int
}

func get_id_mongo() int {
    session, err := mgo.Dial("127.0.0.1")
    if err != nil {
        panic(err)
    }
    defer session.Close()

    // Optional. Switch the session to a monotonic behavior.
    session.SetMode(mgo.Monotonic, true)

    c := session.DB("id_bag").C("id_cookie")
    data := id_cookie{}
    err2 := c.FindId(bson.M{"_id": bson.ObjectIdHex("58593d1d6aace357b32bb3a1")}).One(&data)
    if (err2 != nil){
        Info.Println("error")
        Info.Println(err2)
    }
    Info.Println(data)
    return data.IdCookie
}

Answer: There are two ways to find by id using Collection.FindId() or Collection.Find().

Using Collection.FindId(): Pass only the id value.

err2 := c.FindId(bson.ObjectIdHex("58593d1d6aace357b32bb3a1")).One(&data)

Using Collection.Find(): Specify a value with the field name.

err2 := c.Find(bson.M{"_id": bson.ObjectIdHex("58593d1d6aace357b32bb3a1")}).
    One(&data)

If you still get 0 as the value of id_cookie.IdCookie, it means the field in the document holding this id has a different name. Use struct tags to map it accordingly.

type id_cookie struct {
    IdCookie int `bson:"myid"`
}

To avoid performance issues, connect to the MongoDB server once and reuse the session. For more details, refer to the following thread: [mgo - query performance seems consistently slow (500-650ms)](https://groups.google.com/forum/#!topic/golang-nuts/8_EH7KO_S4Y).

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