[{shopping_cart:[{product_name:"Seven Days",qty:2},{product_name:"Cockta 0.33l",qty:3},{product_name:"Marbo Chips",qty:3},{product_name:"Coca Cola 2l",qty:1}]},{shopping_cart:[{product_name:"Seven Days",qty:2},{product_name:"Coca Cola 2l",qty:1}]},{shopping_cart:[{product_name:"Integral Cookie",qty:4},{product_name:"Seven Days",qty:2},{product_name:"Marbo Chips",qty:1},{product_name:"Coca Cola 2l",qty:4}]}]

db.collection.aggregate([{"$unwind":"$shopping_cart"},{"$group":{"_id":"$shopping_cart.product_name","sum":{"$sum":"$shopping_cart.qty"}}},{"$sort":{sum:-1}},{"$group":{"_id":null,"top_selling_products ":{$push:"$_id"}}}])