Database
[{createdAt:ISODate("2013-10-01T00:00:00.000Z"),qty:12},{createdAt:ISODate("2013-10-05T00:00:00.000Z"),qty:15}]
Query
db.collection.aggregate([{$group:{_id:{month:{$month:"$createdAt"}},qty:{$sum:{$toInt:"$qty"}}}},{$project:{qty:"$qty",Month:{$arrayElemAt:[["","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],"$_id.month"]}}}])