[{title:"product1",price:[{amount:100,currency:"USD",},{amount:20,currency:"EUR",}]},{title:"product2",price:[{amount:330,currency:"USD",},]},{title:"product3",price:[{amount:50,currency:"EUR",},]},]

db.collection.aggregate([{"$unwind":"$price"},{"$group":{"_id":"$price.currency","amount":{"$sum":"$price.amount"}}}])