[{"_id":ObjectId("5e7948fc9a0d0e5ca78aa886"),"product_id":ObjectId("5e76c896eebef71b39aa8277"),"user_qty":[{"user_id":ObjectId("5e76c997eebef71b39aa827a"),"qty":20},{"user_id":ObjectId("5e794aa19a0d0e5ca78aa887"),"qty":40}],"price":20,"reviews":[{"name":"Pablo Perez","comment":"Me quedan muy bien!"},{"name":"Corina Smith","comment":"Muy nice"}],"location":{"street":"La Bonita","city":"Caracas","country":"Venezuela"}}]

db.collection.aggregate([{$addFields:{totalQty:{$sum:"$user_qty.qty"}}}])