使用mgo Golang从MongoDB子文档数组中解组

Question

I am trying to figure out how to get a single subdocument from an array and unmarshal it into a struct.

My mongo document looks like this:

{ 
    "_id" : ObjectId("abc123"), 
    "gamecode" : "abc123"
    "players" : [ 
        { 
            "playerid" : ObjectId("abc123"), 
            "username" : "test", 
        },
        { 
            "playerid" : ObjectId("abc456"), 
            "username" : "test2"
        }]
 }

And I have a player struct that looks like this:

type Player struct {
    PlayerID bson.ObjectId `bson:"playerid" json:"playerid"`
    Username string        `bson:"username" json:"username"`
}

From the mongo command line I can do a

db.games.find(({"players.playerid": ObjectId('abc123')}, {"_id": 0, "players.$":1})

Which returns

{"players" : [{ "playerid" : ObjectId("abc123"), "username" : "test"}]}

But I am having a hard time figuring out how to implement this same functionality in Go so that I have a populated player struct from the result of the query. I have been playing around with different configurations of the code below, but it always results in an empty struct. What am I missing here?

player := Player{}
collection.Find(bson.M{"players.playerid": bson.ObjectIdHex(pid)}).Select(bson.M{"_id": 0, "players.$": 1}).One(&player)

I am running the latest MongoDB version and am using the mgo.v2 driver for Go.

Answer 1

It's because you don't capture a single player, you capture players. like in the response from the mongo command:

{"players" : [{ "playerid" : ObjectId("abc123"), "username" : "test"}]}

Sounds like you can you an abstraction for a game

type Game struct {
    Players []Player `bson:"players"`
}

And your call would be into &game

var game Game
collection.Find(bson.M{"...").One(&game)