mr.js

> db.f.save({u: 1, v: 2})
> db.f.save({u: 2, v: 2})
> db.f.save({u: 1, v: 5})
> db.f.save({u: 2, v: 10})
> m = function() { emit(this.u, this.v) }
function () {
    emit(this.u, this.v);
}
> r = function(k, vs) { r = 0; vs.forEach(function(v) { r += v; }); return r; }
function (k, vs) {
    r = 0;
    vs.forEach(function (v) {r += v;});
    return r;
}
> db.g.mapReduce(m, r, out: 'agg')
> db.agg.find()
{ "_id" : 1, "value" : 7 }
{ "_id" : 2, "value" : 12 }
> db.agg.update({_id: 1}, {$set: {a: 100}})
> db.agg.find()
{ "_id" : 2, "value" : 12 }
{ "_id" : 1, "a" : 100, "value" : 7 }
> db.f.mapReduce(m, r, {out: {reduce: 'agg'}})
{
	"result" : "agg",
	"timeMillis" : 2,
	"counts" : {
		"input" : 4,
		"emit" : 4,
		"reduce" : 2,
		"output" : 3
	},
	"ok" : 1,
}
> db.agg.find()
{ "_id" : null, "value" : null }
{ "_id" : 2, "value" : 24 }
{ "_id" : 1, "value" : 107 }

// "value" should not be 107.