MongoDB: Optimal joining of one to many relationship - mongodb

Question

Here is a hypothetical case of orders and products.
'products' collection
[
{
"_id": "61c53eb76eb2dc65de621bd0",
"name": "Product 1",
"price": 80
},
{
"_id": "61c53efca0a306c3f1160754",
"name": "Product 2",
"price": 10
},
... // truncated
]
'orders' collection:
[
{
"_id": "61c53fb7dca0579de038cea8", // order id
"products": [
{
"_id": "61c53eb76eb2dc65de621bd0", // references products._id
"quantity": 1
},
{
"_id": "61c53efca0a306c3f1160754",
"quantity": 2
},
]
}
]
As you can see, an order owns a list of product ids. When I pull an order's details I also need the product details combined like so:
{
_id: ObjectId("61c53fb7dca0579de038cea8"),
products: [
{
_id: ObjectId("61c53eb76eb2dc65de621bd0"),
quantity: 1,
name: 'Product 1',
price: 80
},
{
_id: ObjectId("61c53efca0a306c3f1160754"),
quantity: 2,
name: 'Product 2',
price: 10
},
... // truncated
]
}
Here is the aggregation pipleline I came up with:
db.orders.aggregate([
{
$match: {_id: ObjectId('61c53fb7dca0579de038cea8')}
},
{
$unwind: {
path: "$products"
}
},
{
$lookup: {
from: 'products',
localField: 'products._id',
foreignField: '_id',
as: 'productDetail'
}
},
{
$unwind: {
path: "$productDetail"
}
},
{
$group: {
_id: "$_id",
products: {
$push: {$mergeObjects: ["$products", "$productDetail"]}
}
}
}
])
Given how the data is organized I'm doubting if the pipeline stages are optimal and could do better (possibility of reducing the number of stages, etc.). Any suggestions?

Answer 1

As already mentioned in comments the design is poor. You can avoid multiple $unwind and $group, usually the performance should be better with this:
db.orders.aggregate([
{ $match: { _id: "61c53fb7dca0579de038cea8" } },
{
$lookup: {
from: "products",
localField: "products._id",
foreignField: "_id",
as: "productDetail"
}
},
{
$project: {
products: {
$map: {
input: "$products",
as: "product",
in: {
$mergeObjects: [
"$$product",
{
$first: {
$filter: {
input: "$productDetail",
cond: { $eq: [ "$$this._id", "$$product._id" ] }
}
}
}
]
}
}
}
}
}
])
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