Why does mgo not return the ID of inserted document?

This should not happen at all, the mgo should insert and return the Id, since, if we generated the ObjectId from the application itself, If the application is restarted, the Object Id generator will start from the beginning generating the same IDs again and again, thus updating existing records in the database.

That is wrong, MGO should rely on the database in generating those IDs and updating the object or returning the objectId of the inserted object immediately like what other languages binding to MongoDB does like in Python or Java.

You use bson.NewObjectId() to generate an ID to be inserted.

This is how you'd insert a new document:

i := bson.NewObjectId()
c.Insert(bson.M{"_id": i, "foo": "bar"})

Since you don't know if you're going to insert or update when you issue an Upsert, it would be superfluous to generate an ID just to drop it right after the query (in case an update happens). That's why it's generated db-side and returned to you when applicable.

You can always try the Upsert function to get the generated ID.

db := service.ConnectDb()
sessionCopy := db.Copy()
defer sessionCopy.Close() // clean up

collection := sessionCopy.DB(service.MongoDB.DTB).C(MessageCol.tbl)

log.Println("before to write: ", msg)

// Update record inserts and creates an ID if wasn't set (Returns created record with new Id)
info, err := collection.Upsert(nil, msg)
if err != nil {
    log.Println("Error write message upsert collection: ", err)
    return MessageMgo{}, err
}

if info.UpsertedId != nil {
    msg.Id = info.UpsertedId.(bson.ObjectId)
}

// gets room from mongo
room, err := GetRoom(msg.Rid)
if err != nil {
    return msg, err
}

// increments the msgcount and update it
room.MsgCount = room.MsgCount + 1
err = UpdateRoom(room)
if err != nil {
    return msg, err
}

return msg, err

This is a sample code I have and works fine.....