MongoDB sum of fields inside objects inside an array that is inside of an object greater than x - mongodb

Question

//8. isbn numbers of books that sold at least X copies (you decide the value for X).
Book example
{
isbn: "0001",
title: "Book1",
pages: NumberInt("150"),
price: NumberDecimal("321.2"),
copies: NumberInt("3"),
language: "english",
author: ["Author1"],
category: ["Space Opera"],
genre: ["Genre-1", "Genre-2"],
character: ["Character-1", "Character-2"],
},
Order example
{
orderNo: "3",
customerNo: "0003",
date: {
day: NumberInt("25"),
month: NumberInt("02"),
year: NumberInt("2021"),
},
orderLine: [
{
isbn: "0006",
price: NumberDecimal("341.0"),
amount: NumberInt("2"),
},
{
isbn: "0007",
price: NumberDecimal("170.5"),
amount: NumberInt("1"),
},
],
},
My try
I believe I have a mistake inside the pipeline at the group stage. For now I need at least to have isbn along with the copies sold in one object.
db.books.aggregate([ // editing this
{ $match : {} },
{
$lookup :
{
from : "orders",
pipeline : [
{
$group :
{
_id: null,
amount_total : { $sum : "$orderLine.amount" }
}
},
{ $project : { _id : 0, amount_total : 1} }
],
as : "amount"
}
},
{ $project : { _id : 0, isbn : 1, amount : 1} }
])
No idea why all are 0's, I was expecting at least some different numbers.
{
"isbn": "0001",
"amount": [
{
"amount_total": 0
}
]
},
{
"isbn": "0002",
"amount": [
{
"amount_total": 0
}
]
},
{
"isbn": "0003",
"amount": [
{
"amount_total": 0
}
]
},// and so on

Answer 1

Apparently, this does what I wanted.
db.books.aggregate([
{
$lookup: {
from: "orders",
let: { isbn: "$isbn" }, // Pass this variable to pipeline for Joining condition.
pipeline: [
{ $unwind: "$orderLine" },
{
$match: {
// Join condition.
$expr: { $eq: ["$orderLine.isbn", "$$isbn"] }
}
},
{
$project: { _id: 0 , orderNo : 1, "orderLine.amount": 1}
}
],
as: "amount"
}
}, { $project : { _id : 0, isbn : 1, amount_total : { $sum : "$amount.orderLine.amount" } } }
])

Answer 2

In your query $lookup is performing a join operation without any condition instead try this query:
db.books.aggregate([
{
$lookup: {
from: "orders",
let: { isbn: "$isbn" },
pipeline: [
{ $unwind: "$orderLine" },
{
$match: {
$expr: { $eq: ["$orderLine.isbn", "$$isbn"] }
}
}
],
as: "amount"
}
},
{
$project: {
_id: 0,
isbn: 1,
amount_total: { $sum: "$amount.orderLine.amount" }
}
}
]);
Test data:
books collection:
/* 1 createdAt:3/12/2021, 10:41:13 AM*/
{
"_id" : ObjectId("604af7f14b5860176c2254b7"),
"isbn" : "0001",
"title" : "Book1"
},
/* 2 createdAt:3/12/2021, 10:41:13 AM*/
{
"_id" : ObjectId("604af7f14b5860176c2254b8"),
"isbn" : "0002",
"title" : "Book2"
}
orders collection:
/* 1 createdAt:3/12/2021, 11:10:51 AM*/
{
"_id" : ObjectId("604afee34b5860176c2254ce"),
"orderNo" : "1",
"customerNo" : "0001",
"orderLine" : [
{
"isbn" : "0001",
"price" : 341,
"amount" : 2
},
{
"isbn" : "0002",
"price" : 170.5,
"amount" : 1
},
{
"isbn" : "0003",
"price" : 190.5,
"amount" : 3
}
]
},
/* 2 createdAt:3/12/2021, 11:10:51 AM*/
{
"_id" : ObjectId("604afee34b5860176c2254cf"),
"orderNo" : "3",
"customerNo" : "0003",
"orderLine" : [
{
"isbn" : "0001",
"price" : 341,
"amount" : 2
},
{
"isbn" : "0002",
"price" : 170.5,
"amount" : 1
},
{
"isbn" : "0003",
"price" : 190.5,
"amount" : 3
}
]
}
Output:
/* 1 */
{
"isbn" : "0001",
"amount_total" : 4
},
/* 2 */
{
"isbn" : "0002",
"amount_total" : 2
}

Answer 3

The $sum inside $group stage will sum root and grouped fields but here orderLine field is an array, you need to sum that array of numbers before applying $sum, it means nested $sum operation,
{
$group: {
_id: null,
amount_total: {
$sum: {
$sum: "$orderLine.amount"
}
}
}
}
Playground
Try the final solution,
$match isbn array in orderLine.isbn using $in condition
$filter to iterate look of orderLine array, and match isbn, it will return filtered documents
$let declare a orders variable to hold above filtered documents of orderLine, sum the amount from filtered array using $sum
$project to show required fields, and get total sum of amount_total array
db.books.aggregate([
{
$lookup: {
from: "orders",
let: { isbn: "$isbn" },
pipeline: [
{ $match: { $expr: { $in: ["$$isbn", "$orderLine.isbn"] } } },
{
$project: {
_id: 0,
amount_total: {
$let: {
vars: {
orders: {
$filter: {
input: "$orderLine",
cond: { $eq: ["$$this.isbn", "$$isbn"] }
}
}
},
in: { $sum: "$$orders.amount" }
}
}
}
}
],
as: "amount"
}
},
{
$project: {
_id: 0,
isbn: 1,
amount_total: { $sum: "$amount.amount_total" }
}
}
])
Playground