MongoDb - mapReduce(...) not working

Question

Sample doc:

{
    "_id" : ObjectId("51cd7274267d959cb9f95cea"),
    "creation_time" : 100,
    "delivered" : true,
    "id" : 1,
    "user_id" : 10
}

Map function:

db.system.js.save({
    _id: "mapDummy",
    value: function(){
        emit(this.user_id,this.user_id);
    }
});

Reduce function:

db.system.js.save({
    _id: "reduceDummy",
    value: function(key,values){
        return Array.sum(values);
    }
});

mapReduce(...) function call:

db.newsdb.mapReduce("mapDummy", "reduceDummy", {out: "notifications_result", query: {delivered:true}});

{
    "result" : "notifications_result",
    "timeMillis" : 16,
    "counts" : {
        "input" : 12,
        "emit" : 0,
        "reduce" : 0,
        "output" : 0
    },
    "ok" : 1,
}

Why emit:0?

Answer 1

I think this is because you use a string and not a function to the call:

db.newsdb.mapReduce("mapDummy", "reduceDummy", {out: "notifications_result", query: {delivered:true}});

Instead of Map/Reduce I would suggest you use the aggregation framework (A/F) anyway. You seem to be doing a simple group by on user_id, which is much better served with A/F as it's a lot faster and easier to use:

db.newsdb.aggregate( { $group: { _id: user_id, count: { $sum: 1 } } } );