This is very similar to group() except group only allows a single key to be extracted per object; map would allow emitting several. And use the map / reduce nomenclature.

Needs to work with mongos too.

db.coll.mapreduce(queryexpr, mapf, reducef);

db.stuff.mapreduce(
{},
function(obj) { obj.tags.forEach(function(t){emit(t,

);} },
function(k,values) {
var res =

;
values.forEach( function(v)

);
emit(k, res);
}