mongodb nested array element $lookup - mongodb

Question

I have two collections, orders and products. I like to join all the order.items[] to products collection to add more fields to the items[]
Sample Data:
orders
[{ _id: 1, items: [
{ product_id: 1, price: 1.99, qty: 2 },
{ product_id: 2, price: 3.99, qty: 5 } ]}]
products
[{ _id: 1, name: "Product 1" }, { _id: 2, name: "Product 2 }]
Expected output:
[{ _id: 1, items: [
{ product_id: 1, name: "Product 1", price: 1.99, qty: 2 },
{ product_id: 2, name: "Product 2",, price: 3.99, qty: 5 } ]}]
I have tried using $lookup and pipeline (mongodb 3.6) and not getting the name value or even the match is not working.
Thanks for a help!

Answer 1

This query will help you, sorry if I didn't use v3.6.
db.orders.aggregate([
{
$unwind: "$items"
},
{
$lookup:
{
from: "products",
localField: "items.product_id",
foreignField: "_id",
as: "tproduct"
}
},
{
$project:
{
"_id" : 1,
"items.product_id" : 1,
"items.name" : { $arrayElemAt: ["$tproduct.name", 0] },
"items.price" : 1,
"items.qty" : 1
}
},
{
$group :
{
_id : "$_id",
items: { $push: "$items" }
}
}
])
They are 4 stages that I will explain:
$unwind will create a single object for each element in the array.
$lookup will find the correct product, keep in mind that Product._id should be unique.
$project will format my documents and in items.name I'm taking the first element of the lookup sentence.
$group will use the _id to group and push each item into a new array.
I'm pretty sure there are cleaner and easier ways to write this, but this should work without problems.