Question

I am trying to figure out this map/reduce system in mongoDB. I have the following basic schema/layout in my collection.

{
    _id: 1,
    name: n1,
    website: w1,
    tags: [
        myTag1,
        myTag3
    ]
}

{
    _id: 2,
    name: n2,
    website: w2,
    tags: [
        myTag2,
        myTag3
    ]
}

{
    _id: 3,
    name: n3,
    website: w3,
    tags: [
        myTag2,
        myTag4
    ]
}

How can I retrieve an array of unique tags? I would like this to be returned to me for further use.

{
    tags: [
        myTag1,
        myTag2,
        myTag3,
        myTag4
    ]
}

By the way this is what I have come up with, but it just returns the _id and tags of each item instead of combining the tags into a single object.

var map = function() {emit( this._id,{tags: this.tags});};

var reduce = function(key, values) {
    var t = [];

    values.forEach(function(doc) {
        var tags = doc.tags;
        tags.forEach(function(tag) {
            if (!(tag in t)) {
                t.push(tag);
            }
        });
    });

    return {tags: t};
};

var op = db.businesses.mapReduce(map, reduce, {out: "mr_results"});

db[op.result].find();

Answer 1

There’s no need to use map-reduce in your case. Just use the distinct function:

db.businesses.distinct('tags')

You can try it in the mongo shell:

> use test
switched to db test
> db.businesses.insert({tags: ['tag1', 'tag2']})
> db.businesses.insert({tags: ['tag3', 'tag4']})
> db.businesses.find()
{ "_id" : ObjectId("4fa05b2b036495bf4ac9c0cc"), "tags" : [ "tag1", "tag2" ] }
{ "_id" : ObjectId("4fa05b33036495bf4ac9c0cd"), "tags" : [ "tag3", "tag4" ] }

> db.businesses.distinct('tags')
[ "tag1", "tag2", "tag3", "tag4" ]

Also, you should keep in mind that map/reduce in MongoDB is not suitable for real-time querying.