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

db.order.aggregate([{"$match":{"orderId":"2001"}},{"$lookup":{"from":"item",let:{eid:"$itemId"},pipeline:[{"$match":{$expr:{$in:["$_id","$$eid"]}}},],"as":"items"}},{$project:{orderId:1,finalSum:{$subtract:[{$sum:{$map:{input:"$items",in:"$$this.Price"}}},"$Discount"]}}}])