I am trying to run a map/reduce function in mongodb where I group by

Question

0

Asked: June 6, 20262026-06-06T22:56:14+00:00 2026-06-06T22:56:14+00:00

I am trying to run a map/reduce function in mongodb where I group by

0

I am trying to run a map/reduce function in mongodb where I group by 3 different fields contained in objects in my collection. I can get the map/reduce function to run, but all the emitted fields run together in the output collection. I’m not sure this is normal or not, but outputting the data for analysis takes more work to clean up. Is there a way to separate them, then use mongoexport?

Let me show you what I mean:

The fields I am trying to group by are the day, user ID (or uid) and destination.

I run these functions:

map = function() {
    day = (this.created_at.getFullYear() + "-" + (this.created_at.getMonth()+1) + "-" + this.created_at.getDate());
    emit({day: day, uid: this.uid, destination: this.destination}, {count:1});
}

/* Reduce Function */
reduce = function(key, values) { 
    var count = 0;
    values.forEach(function(v) {
        count += v['count'];
}
);
  return {count: count};
}

/* Output Function */
db.events.mapReduce(map, reduce, {query: {destination: {$ne:null}}, out: "TMP"});

The output looks like this:

{ "_id" : { "day" : "2012-4-9", "uid" : "1234456", "destination" : "Home" }, "value" : { "count" : 1 } }
{ "_id" : { "day" : "2012-4-9", "uid" : "2345678", "destination" : "Home" }, "value" : { "count" : 1 } }
{ "_id" : { "day" : "2012-4-9", "uid" : "3456789", "destination" : "Login" }, "value" : { "count" : 1 } }
{ "_id" : { "day" : "2012-4-9", "uid" : "4567890", "destination" : "Contact" }, "value" : { "count" : 1 } }
{ "_id" : { "day" : "2012-4-9", "uid" : "5678901", "destination" : "Help" }, "value" : { "count" : 1 } }

When I attempt to use mongoexport, I can not separate day, uid, or destination by columns because the map combines the fields together.

What I would like to have would look like this:

{ { "day" : "2012-4-9" }, { "uid" : "1234456" }, { "destination" : "Home"}, { "count" : 1 } }

Is this even possible?

As an aside – I was able to make the output work by applying sed to the file and cleaning up the CSV. More work, but it worked. It would be ideal if I could get it out of mongodb in the correct format.