[{"productCategory":"category1","views":1},{"productCategory":"category1","views":10},{"productCategory":"category1","views":2},{"productCategory":"category2","views":1},{"productCategory":"category2","views":2},{"productCategory":"category2","views":4},{"productCategory":"category3","views":10},{"productCategory":"category3","views":10},]

db.collection.aggregate([{"$group":{"_id":"$productCategory","views":{"$sum":"$views"}}},{"$sort":{"views":-1}}])