mgo convert mapreduce to aggregations command

Question

I am trying to convert this function here from mapreduce to aggregation.

https://github.com/mendersoftware/deviceauth/blob/be2f9745e14bbe87121d99ac6c396f41ca7438e2/store/mongo/datastore_mongo.go#L826

The result from above function will be [{pending 1}].

When I run my match group I get [{pending 0}] from the following code below:

    mat := bson.M{
            "$match": bson.M{"device_id": devId},
    }

    grp := bson.M{
            "$group": bson.M{
                    "_id": "$status",
                    "count": bson.M{
                            "$sum": 1,
                    },
            },
    }

    pipe := c.Pipe([]bson.M{mat,grp})
    pipe.One(&result)

But the same I think command in mongo shell gives [{pending 1}].

db.getCollection("auth_sets").aggregate([
{
    $match: {
        device_id:"5c79601d152ece00012f5831"
    }
},
{
    $group: {
        _id:"$status",
        count: {
            $sum: 1
        }
     }
},
]);

How can I get it so my pipe will return [{pending 1}]?

I am changing it so I can use Mongo Atlas with does not allow mapreduce.

Answer 1

Your mgo query structure is OK, the problem is the name of the count field. The model expects Value:

var result []struct {
    Status string `bson:"_id"`
    Value  int
}

So change the $group stage to this:

grp := bson.M{
        "$group": bson.M{
                "_id": "$status",
                "value": bson.M{   // Note lowercased "value"!
                        "$sum": 1,
                },
        },
}

And it should work. Or change the model if you can:

var result []struct {
    Status string `bson:"_id"`
    Value  int    `bson:"count"`
}

Only one of them needs to be changed, to be aligned with the other.

One last thing: If you use Query.One(), then the result must not be a slice (One() expects one document exactly).

Use a slice type for result if you use e.g. Query.All().

So if you're going to use Query.One(), use result:

var result struct {
    Status string `bson:"_id"`
    Value  int    `bson:"count"`
}

Also Query.One() and Query.All() return an error, do check it!