count dups in mongo with mapReduce

mongo_dups.js

//count dups in mongo with mapReduce
m = function() { emit(this.user_id, 1); };
r = function (k, vals) { return Array.sum(vals); };
res = db.tasks.mapReduce(m, r, { out : {replace : "results", db : "mr"}});

// more difficult 
function map() {
    var sorted = this._keywords.sort();
    key = sorted.join(" ");
    var ret = {title : this.title, id : this._id, total: 1};
    emit(key, ret);
}

function reduce (k, values) {
    var ret = { total: 0, titles: new Array(), ids: new Array() };
    values.forEach(function(value) {
        ret.total += value.total;
        ret.titles.push(value.title);
        ret.ids.push(value.id);
    });
    return ret;
}
res = db.books_to_buy.mapReduce(map, reduce, { out : {replace : "results", db : "ltl"}});

> use mr
> db.results.find().sort({'values' : -1})