Unable to aggregate two collections using lookup in MongoDB Atlas - mongodb

Question

I have an orders collection that looks like this:
{
"_id" : "wJNEiSYwBd5ozGtLX",
"orderId" : 52713,
"createdAt" : ISODate("2020-01-31T04:34:13.790Z"),
"status" : "closed",
"orders" : [
{
"_id" : "ziPzwLuZrz9MNkaRT",
"productId" : 10290,
"quantity" : 2
}
]
}
I have an products collection that looks like this
{
"_id" : "238cwwLkZa6gKNN86",
"productId" : 10290,
"title" : "Product Title",
"price" : 9.9
}
I am trying to merge the price information into the orders information.
Something like:
{
"_id" : "wJNEiSYwBd5ozGtLX",
"orderId" : 52713,
"createdAt" : ISODate("2020-01-31T04:34:13.790Z"),
"status" : "closed",
"orders" : [
{
"_id" : "ziPzwLuZrz9MNkaRT",
"productId" : 10290,
"quantity" : 2,
"price": 9.9
}
]
}
If I try a $lookup command on MongoDB Atlas Dashboard like this:
{
from: 'products',
localField: 'orders.productId',
foreignField: 'productId',
as: 'priceInfo'
}
The aggregated output is (not what I wanted):
{
"_id" : "wJNEiSYwBd5ozGtLX",
"orderId" : 52713,
"createdAt" : ISODate("2020-01-31T04:34:13.790Z"),
"status" : "closed",
"orders" : [
{
"_id" : "ziPzwLuZrz9MNkaRT",
"productId" : 10290,
}
],
"priceInfo": [
{
"_id" : "238cwwLkZa6gKNN86",
"productId" : 10290,
"title" : "Product Title",
"price" : 9.9
}
]
}
I do not need a separate priceInfo array. It will be best if I have the product details information merged into the "orders" array. What should be the aggregation lookup syntax to achieve the desired output?

Answer 1

Demo - https://mongoplayground.net/p/bLqcN7tauWU
Read - $lookup $unwind $first $set $push $group
db.orders.aggregate([
{ $unwind: "$orders" }, // break array of orders into individual documents
{
$lookup: { // join
"from": "products",
"localField": "orders.productId",
"foreignField": "productId",
"as": "products"
}
},
{
$set: {
"orders.price": { "$arrayElemAt": [ "$products.price", 0 ] } // set the price
}
},
{
$group: { // group records back
_id: "$_id",
createdAt: { $first: "$createdAt" },
status: { $first: "$status" },
orderId: { $first: "$orderId" },
orders: { $push: "$orders" }
}
}
])