CODEWITHSUNDEEP
mongodb
user974802
user974802

Reputation: 3457

MongoDB : Calculating wrongly in mapReduce

This is my collection named logins

db.logins.find().pretty()


      "cust_id" : "testuser",
       "created_at" : "2011-03-11 10:31:02.765"

      "cust_id" : "testuser",
      "created_at" : "2011-03-11 10:31:02.765"

I am trying to find out Logins of each user based on a Date

I tried this way 

var m = function() {
    var aux = this.created_at.indexOf(' ');
    emit({cust:this.cust_id,daterr:this.created_at.substring(0,aux)},1);
}

var r = function(k, values) {
  var l = values.length;
  var r = 0;
  var i;
  for (i=0; i<l; i++) {
    r+=l;
  }
  return r;
}

 q = function() {
        var query = {"created_at": {$lt: "2013-04-30 11:19:52.587"}}
        return query;
    }

db.logins.mapReduce(m, r, { query : q(), out :  "userLoginCountMonthly" });

I am getting the following Output 

db.userLoginCountMonthly.find().pretty()

{ "_id" : { "cust" : "testuser", "daterr" : "2011-03-11" }, "value" : 4 }

What i see is that the value must be 2 , but it is shown as 4

Could you please tell me why its displaying wrong ??

Upvotes: 0

Views: 60

Answers (1)

Joachim Isaksson
Joachim Isaksson

Reputation: 180917

I may very well be totally off in this since I've never used MongoDB's mapreduce and I'm not quite sure what you're trying to achieve in the reduce, but this looks wrong;

var l = values.length;
var r = 0;
var i;
for (i=0; i<l; i++) {
  r+=l;
}
return r;

If values.length is 2, this will loop twice, each time adding 2, returning 4.

As far as I can see since you only want a count, a simple return values.length would make more sense as a reduce function.

Upvotes: 1

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