CODEWITHSUNDEEP
mongodbmapreduce
Dreamwalker
Dreamwalker

Reputation: 3035

Struggling with a map-reduce in MongoDb

Problem

I have a document with a _id and a Collection of Answers I am trying to write a map-reduce function to sum the total score of answers for each id.

Document

/* 0 */
{
  "_id" : ObjectId("527b6ba88d251d58a18f3f0a"),
  "Answers" : [{
      "Score" : 2
    }, {
      "Score" : 0
    }, {
      "Score" : 2
    }, {
      "Score" : 2
    }]
}

Here is the Map-Reduce I though would be correct reading the documentation

Map

function() {
    this.Answers.forEach(function(val)
    { 
        emit(this._id, val.Score); 
    });
}

also tried this

function() {

    for (var i = 0; i < this.Answers; i++)
    { 
        emit(this._id, this.Answers[i].Score); 
    }
}

Reduce

function(key, values) 
{
    return Array.sum(values);
}

I am getting no information back with this, but it does appear to be processing it takes 2-5 seconds to return. I guess I am not understanding something about map-reduce.

Also I am using MongoVUE to access MongoDB.

EDIT

I just ran my map reduce through the console and got this output

{
        "results" : [ ],
        "timeMillis" : 2506,
        "counts" : {
                "input" : 1655,
                "emit" : 0,
                "reduce" : 0,
                "output" : 0
        },
        "ok" : 1,
}

so it's my map function that's incorrect I guess as nothing was emitted.

EDIT 2

Updated document with output from mongovue

Upvotes: 0

Views: 154

Answers (2)

Adam Pridmore
Adam Pridmore

Reputation: 163

I think the 'this' variable is not what you expect in the .forEach() function in your map method. Try this instead;

function() {
    var row = this;
    this.Answers.forEach(function(val)
    { 
      emit(row._id, val.Score); 
    });
}

Upvotes: 1

Sammaye
Sammaye

Reputation: 43884

In JavaScript loops adding the length property allows you to iterate by the count of the items in the array, so you cna change your second attempt to:

function() {

    for (var i = 0; i < this.Answers.length; i++)
    { 
        emit(this._id, this.Answers[i].Score); 
    }
}

It should also be noted that your reduce can run multiple times per key, specifically it can repeat every 101 rows, technically this shouldn't matter since you are summing up the array values and the previous reduce value will be passed as an array element in the new reduce so it should work just fine; however, good to keep in mind.

Upvotes: 1

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