Question

I am developing a web app using Codeigniter and MongoDB.
I am trying to get the map reduce to work.

I got a file document with the below structure. I would like to do a map reduce to
check how many times each tag is being used and output it to the collection files.tags.

{
    "_id": {
        "$id": "4f26f21f09ab66c1030d0000e"
    },
    "basic": {
        "name": "The filename"
    },
    "tags": [
        "lorry",
        "house",
        "car",
        "bicycle"
    ],
    "updated_at": "2012-02-09 11:08:03"
}

I tried this map reduce command but it does not count each individual tag:

$map = new MongoCode ("function() {

 emit({tags: this.tags}, {count: 1});

}");

    $reduce = new MongoCode ("function( key , values ) {

     var count = 0;

     values.forEach(function(v) {
      count += v['count'];
     });

     return {count: count};

    }");

    $this->mongo_db->command (array (

     "mapreduce" => "files",   
     "map"       => $map,   
     "reduce"    => $reduce,   
     "out"       => "files.tags"

    )

    );

Answer 1

Yea, this map/reduce simply calculate total count of tags.

In mongodb cookbook there is example you are looking for.

You have to emit each tag instead of entire collection of tags:

map = function() {
    if (!this.tags) {
        return;
    }

    for (index in this.tags) {
        emit(this.tags[index], 1);
    }
}