> db.b.insert({k:{a:2,b:1},v:567})
> db.b.insert({k:{a:1,b:1},v:111})
> db.b.insert({k:{a:1,b:2},v:12})
> db.b.insert({k:{a:2,b:2},v:22})
> db.b.aggregate({$group:{_id:'$k.a', s:{$sum:'$v'}}})
{
        "result" : [
                {
                        "_id" : 1,
                        "s" : 123
                },
                {
                        "_id" : 2,
                        "s" : 589
                }
        ],
        "ok" : 1
}

> db.a.insert({_id:{a:1,b:1},v:123})
> db.a.insert({_id:{a:1,b:2},v:345})
> db.a.insert({_id:{a:2,b:2},v:222})
> db.a.insert({_id:{a:2,b:1},v:567})
> db.a.aggregate({$group:{_id:'$_id.a', s:{$sum:'$v'}}})
{ "result" : [ { "_id" : null, "s" : 1257 } ], "ok" : 1 }