MongoDB agregation and comparing - mongodb

Question

I need to write mongodb query that will agregate items by quantinty (orders collestion). After agregation, or with agregation, it should identify items that can't be supplied in stock collection, and return name of the person who made this order
code of orders insert
db.orders.insertMany([
{client: {firstName: "John",lastName: "Smith" },orderedItems: [{ name: "itemA", qty: 5},{ name: "itemS", qty: 3}]},
{client: {firstName: "Jan",lastName: "Nowak" },orderedItems: [{ name: "itemA", qty: 56},{ name: "itemS", qty: 53}]},
{client: {firstName: "Klara",lastName: "Bolączka" },orderedItems: [{ name: "itemA", qty: 35},{ name: "itemS", qty: 23}]},
{client: {firstName: "Brajan",lastName: "Kowalski" },orderedItems: [{ name: "itemA", qty: 95},{ name: "itemS", qty: 13}]}
]);
code of stocks insert
db.stock.insertMany([
{ item: "itemA", qty: 40},
{ item: "itemS", qty: 113}
]);
Right now i got some mongoDB code, that just agregates orders
db.orders.aggregate([
{$unwind: "$orderedItems"},
{$group: { _id: "$orderedItems.name", total:{$sum: "$orderedItems.qty"}}}
]);
I tried to use lookup, but probably I'm doing something wrong.
Could I get some sugestions, please?

Answer 1

How would you like something like this:
db.orders.aggregate([
{
$unwind: "$orderedItems" // flatten the "orderedItems" array
},
{
$group: {
"_id": "$orderedItems.name", // group by orderItems.name
"total": {
$sum: "$orderedItems.qty" // calculate total quantity
},
"users": {
$addToSet: "$client" // create an array of clients who have ordered each item
}
}
}, {
$lookup: { // retrieve the stock information
from: "stock",
localField: "_id",
foreignField: "item",
as: "stock"
}
}, {
$unwind: "$stock" // flatten the result array - should always return only one document, given your data model
}, {
$addFields: { // add a new field...
"difference": { // ...called "difference"...
$subtract: [ "$stock.qty", "$total" ] // ...in which we store the difference between the stock qty and the ordered qty
}
}
}, {
$match: { // only return documents...
"difference": { // ...where the value for "difference"...
$lt: 0 // ...is less than zero
}
}
}])