db={"orders":[{"_id":ObjectId("615fc295257d6d7cf57a39fe"),"orderId":"2001","itemId":[ObjectId("615fc232257d6d7cf57a39d4"),ObjectId("615fc251257d6d7cf57a39e0")],"Discount":10}],"items":[{"_id":ObjectId("615fc232257d6d7cf57a39d4"),"itemId":"1001","Price":10.21},{"_id":ObjectId("615fc251257d6d7cf57a39e0"),"itemId":"1002","Price":100}]}

db.orders.aggregate([{"$match":{"orderId":"2001"}},{"$lookup":{"from":"items","localField":"itemId","foreignField":"_id","as":"items"}},{"$project":{"orderId":1,"total":{$subtract:[{"$sum":"$items.Price"},"$Discount"]}}}])