Adding remaining filed in $project after adding lookups in mongoose - mongodb

Question

I'm doing a $lookup from an _id in Order schema, and its working as expected. But in $project how to add remaining keys. I have added my code below.
Product Collection:
{
"_id": "54759eb3c090d83494e2d804",
"product_name": "sample product",
"image": "default.png",
"price": 55,
"discount": 5,
}
Order list Collection
{
"user_name": "sample1",
"product_list":[
{
"product_id": "54759eb3c090d83494e2d804"
"quantity": 5
}
]
}
lookups
[
{
from: 'product',
localField: 'product_list.product_id',
foreignField: '_id',
as: 'product_list.product_id',
model: 'ProductModel',
},
],
$Project
{
user_name: true,
product_list: {
$map: {
input: '$product_list.product_id',
as: 'product',
in: {
product_name: '$$product.product_name',
},
},
},
}
Current Result:
{
"user_name": "sample1",
"product_list":[
"product_id":{
"product_name": "sample product"
}
]
}
In this current result, the quantity field is missing. How to add in $project?. The expected result shown below
Expected Result:
{
"user_name": "sample1",
"product_list":[
{
"product_id": {
"product_name": "sample product"
}
"quantity": 5
}
]
}

Answer 1

You need to do $unwind before $lookup, because it will not work directly in array fields, and here you don't need $map inside $project,
$unwind product_list deconstruct array
db.order.aggregate([
{ $unwind: "$product_list" },
$lookup with pipeline, this will allow to use pipeline inside lookup, here $project to required fields
{
$lookup: {
from: "product",
as: "product_list.product_id",
let: { product_id: "$product_list.product_id" },
pipeline: [
{
$match: {
$expr: { $eq: ["$$product_id", "$_id"] }
}
},
{
$project: {
_id: 0,
product_name: 1
}
}
]
}
},
$unwind with path product_list.product_id because you need it as object
{ $unwind: { path: "$product_list.product_id" } },
$group by _id re-construct your product_list array
{
$group: {
_id: "$_id",
user_name: { $first: "$user_name" },
product_list: { $push: "$product_list" }
}
}
])
Playground