CODEWITHSUNDEEP
mongodbmapreduce
Amit
Amit

Reputation: 34725

MongoDb - mapReduce(...) not working

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?

Upvotes: 0

Views: 993

Answers (2)

Jason Mullings
Jason Mullings

Reputation: 994

After an hour of failed attempts...!

let maps = function() {
          emit(this. user_id, 1)
        };
let reduces = function(keys, vals){
          return Array.sum(vals);
        };

result.collection.mapReduce(maps, reduces, { out: { reduce: 'test_collection' 
}, jsMode: true, verbose: true})
.then(res=> console.log(res.stats))
.catch(err=> console.log({err}));

..I established that 'mapReduce' does not accept 'Fat Arrows' =>=>=>=>|_ Maps || Reduces!!

Upvotes: 0

Derick
Derick

Reputation: 36764

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 } } } );

Upvotes: 2

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