golang mongodb组聚合返回伪记录

Question

I am trying fetch documents from mongdb using golang code base. I created a pipeline and am using aggregate function like this:

pipeline := []bson.M{
        bson.M{
            "$group": bson.M{"_id": "$user_id", "score": bson.M{"$max": "$score"}},
        },
        bson.M{
            "$sort": bson.M{"score": 1},
        },
        bson.M{
            "$limit": 10,
        },
    }

collection := dBClient.Database("db").Collection("collection")
iter, err := collection.Aggregate(context.Background(), pipeline)
var sc score
for iter.Next(context.Background()) {
        iter.Decode(&sc)
        if err != nil {
            log.Fatal(err)
        }
        //print element data from collection
        fmt.Printf("Element %v", sc)
    }

    type score struct {
      ID        primitive.ObjectID `bson:"_id,omitempty"`
      UserID    string             `bson:"user_id"`
      Score     int32              `bson:"score"`
      Timestamp int64              `bson:"timestamp"`
    }

but all i am getting is bunch of zeros

Element {ObjectID("000000000000000000000000")  0 0}
Element {ObjectID("000000000000000000000000")  0 0}
Element {ObjectID("000000000000000000000000")  0 0}
Element {ObjectID("000000000000000000000000")  0 0}
Element {ObjectID("000000000000000000000000")  0 0}
Element {ObjectID("000000000000000000000000")  0 0}
Element {ObjectID("000000000000000000000000")  0 0}
Element {ObjectID("000000000000000000000000")  0 0}
Element {ObjectID("000000000000000000000000")  0 0}

same query runs fine on robo3t

db.getCollection('collection').aggregate([
    {"$group":
         {_id:"$user_id",
         score:{"$max":"$score"}}},
    {"$sort":{"score":-1}},
    {"$limit":10}
])

although when i don't use '$group' it works fine and gives me proper records.

pipeline := []bson.M{
        bson.M{
            "$sort": bson.M{"score": 1},
        },
        bson.M{
            "$limit": 10,
        },
    }

what am i missing?

i am using go.mongodb.org/mongo-driver/mongo driver