Getting null array as result in mongodb aggregate pipeline - node.js

Question

Here is my code, I have two collections called cart and product, First I want to take the id of the product from the cart and then from the product collection get the product details
collection cart
{
"_id" : ObjectId("62f8086e29c549f34ab89df7"),
"user" : ObjectId("62f3f8600e93c17d1c25c2ed"),
"product" : [
ObjectId("62f391b9482a375c4f83de8e"),
ObjectId("62f39121482a375c4f83de8d"),
ObjectId("62f39200482a375c4f83de8f")
]
}
collection product
{
"_id" : ObjectId("62f39121482a375c4f83de8d"),
"name" : "iphone 15",
"category" : "mobiles",
"price" : "125",
"description" : "fastest iphone"
}
{
"_id" : ObjectId("62f391b9482a375c4f83de8e"),
"name" : "OnePlus Nord",
"category" : "mobile",
"price" : "40000",
"description" : "budget phone of OnePlus"
}
{
"_id" : ObjectId("62f39200482a375c4f83de8f"),
"name" : "Samsung M33",
"category" : "mobile",
"price" : "25000",
"description" : "mid range mobile"
}
Here is the function for getting the details
getcartProducts:(userId)=>{
return new Promise(async(resolve,reject)=>{
let cartItem=await db.get().collection(collection.CART_COLLECTION).aggregate([
{
"$match":{user:objectId(userId)}
},
{
"$lookup":{
"from":collection.PRODUCT_COLLECTIONS,
"let":{"prodList":'$product'},
pipeline:[
{
"$match":{ "$expr":{ "$in":["_id","$$prodList"],
},
},
},
],
"as":"cartItems"
}
}
]
).toArray()
resolve(cartItem)
})
}
Finally the function is called
router.get('/cart',verifyLogin,async (req,res)=>{
let products=await userHelpers.getcartProducts(req.session.user._id)
console.log(products)
res.render('user/cart')
})
The output is: Showing null array in cartItems: []
[
{
_id: new ObjectId("62f8086e29c549f34ab89df7"),
user: new ObjectId("62f3f8600e93c17d1c25c2ed"),
product: [
new ObjectId("62f391b9482a375c4f83de8e"),
new ObjectId("62f39121482a375c4f83de8d"),
new ObjectId("62f39200482a375c4f83de8f"),
new ObjectId("62f39121482a375c4f83de8d")
],
cartItems: []
}
]

Answer 1

Use $_id instead of _id in $in operator. Mongo Playground
// This is the raw query
db.carts.aggregate([
{
"$match": {
"user": ObjectId("62f3f8600e93c17d1c25c2ed")
}
},
{
"$lookup": {
"from": "products",
"as": "cartItems",
"let": {
"prodList": "$product"
},
pipeline: [
{
"$match": {
"$expr": {
"$in": [
"$_id", // note the change here, use $ for matching field in lookup collection
"$$prodList" // use $$ for matching variable
],
},
},
},
],
}
}
])
Change the following line in your example code:
"$match":{ "$expr":{ "$in":["_id","$$prodList"],
To:
"$match":{ "$expr":{ "$in":["$_id","$$prodList"],
This aggregation pipeline cane be simplified to: Mongo Playground
[
{
"$match": {
"user": ObjectId("62f3f8600e93c17d1c25c2ed")
}
},
{
"$lookup": {
"from": "products",
"as": "cartItems",
"localField": "product",
"foreignField": "_id"
}
}
]