mgo中的Golang和MongoDB查询

Question

So I have this mongo db query which works fine:

db.levels.aggregate([{
         $match: {
                  "_id": {$lt: ObjectId("56410480f91e505237902dae")}
         },
},     
        { $group: 
            {             
                 "_id": {"title": "Level 11"},
                 "totalAmount": { $sum: "$rewardCoins"}
            }
        } 
])

It should get all rows before give id and based on rewardCoins calculate sum.

Now with mgo in golang I'm strugling to get this working... I'm going with pipe but my resultset is empty.

pipe := c.Pipe([]bson.M{{"$match": bson.M{"_id": bson.M{"$lt": level.ID}},
        "$group": bson.M{"_id":"$title", "totalReward": bson.M{"$sum": "$rewardCoins"}}}})

    res :=[]bson.M{}

    pipe.All(&res)

What I'm doing wrong here ? Thanks.

UPDATE

Here is how my simple rows look like in db:

{ "_id" : ObjectId("5613f5ad153678d113d01f4a"), "title" : "Level 1", "rewardCoins" : NumberLong(1000) }
{ "_id" : ObjectId("56159796153678d113d02d60"), "title" : "Level 2", "rewardCoins" : NumberLong(50000) }

Answer 1

The structure in Go isn't the same as in the other language. If you hit Enter after every brace or comma, add commas where Go wants them, and run go fmt, you get one bson.M:

pipe := c.Pipe(
    []bson.M{
        {
            "$match": bson.M{
                "_id": bson.M{
                    "$lt": level.ID,
                },
            },
            "$group": bson.M{
                "_id": "$title",
                "totalReward": bson.M{
                    "$sum": "$rewardCoins",
                },
            },
        },
    },
)

Another way to see it is that there's no }, { before $group in the Go, whereas there is in the JSON-y version.

Indenting this way seems like a good idea in general; the eyes can get lost in a sea of brackets otherwise.