[{"customer":"21c3","shopDate":new Date("2019-06-03T23:00:00Z"),"shopId":"Supermarket","items":[{"productName":"Water","price":3,"quantity":2},{"productName":"Candies","price":1,"quantity":5}]}]

db.collection.aggregate([{"$project":{"items":{"$sum":{"$map":{"input":"$items","in":{"$multiply":["$$this.price","$$this.quantity"]}}}}}}])