How to use `$unwind` to iterate exact value - mongodb

Question

I am new to mongoose and mongodb.
In my addtocart schema I have added $lookups and projection to populate products in add to cart.
In current response product_purchase_quantity it was in array in add to cart collection it is key and value. So, in $lookups I tried to add {$unwind: '$product_purchase_quantity'} but after adding add_to_cart_products object prints twice. I don't know where the mistake was?
Below i have mentioned expected result.
AddToCart Schema:
lookups: [
{
from: 'shop_db_products',
let: {
productId: '$add_to_cart_products.product',
purchaseQuantity: '$add_to_cart_products.product_quantity',
productItemId: '$add_to_cart_products.product_item',
},
pipeline: [
{
$match: { $expr: { $in: ['$_id', '$$productId'] } },
},
{
$lookup: {
from: 'shop_db_products',
localField: 'product_id',
foreignField: '_id',
as: 'products',
},
},
{
$project: {
_id: true,
product: {
_id: '$_id',
product_name: '$product_name',
},
product_purchase_quantity: '$$purchaseQuantity',
product_item: {
$reduce: {
input: {
$filter: {
input: '$product_items',
cond: {
$in: ['$$this._id', '$$productItemId'],
},
},
},
initialValue: {},
in: {
_id: '$$this._id',
product_size: { $concat: [{ $toString: '$$this.product_size.value' }, '$$this.product_size.unit'] },
product_price: '$$this.product_price',
product_type: '$$this.product_type'
},
},
},
},
},
],
localField: '',
as: 'add_to_cart_products',
model: 'ProductModel',
},
],
AddToCart Collection
[
{
"add_to_cart_user": "5f0076b7bd530928fc0c0285",
"add_to_cart_products": [
{
"product": "5f05a0270b4f3a5c41c70826",
"product_item": "5f05a0270b4f3a5c41c70877",
"product_quantity": 5
},
{
"product": "5f05a0270b4f3a5c41c70827",
"product_item": "5f05a0270b4f3a5c41c70666",
"product_quantity": 3
}
],
"add_to_cart_product_total": 5,
"add_to_cart_discount": 50,
"add_to_tax": "5eae321d21924800122f978e",
"add_to_cart_grand_total": 500
}
]
Current Response:
{
"_id": "5fa2a09b3c6316482098f6ff",
"add_to_cart_status_is_active": true,
"add_to_cart_discount": 50,
"add_to_cart_tax": 8,
"add_to_cart_products": [
{
"product": {
"_id": "5f05a0270b4f3a5c41c70826",
"product_name": "Avery Apricot Sour"
},
"product_purchase_quantity": [
5,
3
],
"product_item": {
"_id": "5f05a0270b4f3a5c41c70877",
"product_size": "22oz",
"product_price": 13.99
}
},
{
"product": {
"_id": "5f05a0270b4f3a5c41c70827",
"product_name": "Avery Dugana"
},
"product_purchase_quantity": [
5,
3
],
"product_item": {
"_id": "5f05a0270b4f3a5c41c70666",
"product_size": "22oz",
"product_price": 8.99
}
}
]
}
Expected response:
{
"_id": "5fa2a09b3c6316482098f6ff",
"add_to_cart_status_is_active": true,
"add_to_cart_discount": 50,
"add_to_cart_tax": 8,
"add_to_cart_products": [
{
"product": {
"_id": "5f05a0270b4f3a5c41c70826",
"product_name": "Avery Apricot Sour"
},
"product_purchase_quantity": 5,
"product_item": {
"_id": "5f05a0270b4f3a5c41c70877",
"product_size": "22oz",
"product_price": 13.99
}
},
{
"product": {
"_id": "5f05a0270b4f3a5c41c70827",
"product_name": "Avery Dugana"
},
"product_purchase_quantity": 3,
"product_item": {
"_id": "5f05a0270b4f3a5c41c70666",
"product_size": "22oz",
"product_price": 8.99
}
}
]
}
Product (shop_db_products) collection:
[
{
"_id": "5f05a0270b4f3a5c41c70826",
"product_no": "PRO04087",
"product_store_no": "1001",
"product_dept_no": "Irish Whiskey",
"product_name": "2Gingers",
"product_overview": "No Overview",
"product_items": [
{
"_id": "5f05a0270b4f3a5c41c70877",
"product_item_number": "857566003019",
"product_price": 20.99,
"product_cost": 20.99,
"product_size": "750ml",
"product_type": "Bottle",
"product_value": 0,
"product_quantity": 0,
"product_images": [
"pro04087-857566003019-1.png"
],
"product_item_is_active": true
}
]
},
...
]

Answer 1

You have to $unwind add_to_cart_products before performing $lookup
db.addToCart.aggregate([
{
$unwind: "$add_to_cart_products"
},
{
$lookup: {
from: "shop_db_products",
let: {
productId: "$add_to_cart_products.product",
purchaseQuantity: "$add_to_cart_products.product_quantity",
productItemId: "$add_to_cart_products.product_item"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$_id",
"$$productId"
]
}
}
},
{
$project: {
_id: true,
product: {
_id: "$_id",
product_name: "$product_name",
},
product_purchase_quantity: "$$purchaseQuantity",
product_item: {
$reduce: {
input: {
$filter: {
input: "$product_items",
cond: {
$eq: [
"$$this._id",
"$$productItemId"
],
},
},
},
initialValue: {},
in: {
_id: "$$this._id",
product_size: "$$this.product_size",
product_price: "$$this.product_price",
product_type: "$$this.product_type"
}
}
}
}
}
],
as: "add_to_cart_products"
}
},
{
$unwind: "$add_to_cart_products"
},
{
$group: {
_id: "$_id",
"add_to_cart_products": {
$push: "$add_to_cart_products"
}
}
}
])
MongoDB Playground

Answer 2

Another way
Without reducer, with $unwind
https://mongoplayground.net/p/3uWA5pVBv83
db.addToCart.aggregate([
{
"$unwind": "$add_to_cart_products"
},
{
"$lookup": {
"from": "shop_db_products",
"let": {
"productId": "$add_to_cart_products.product",
"purchaseQuantity": "$add_to_cart_products.product_quantity",
"productItemId": "$add_to_cart_products.product_item"
},
"pipeline": [
{
"$match": {
"$expr": {
"$eq": [
"$_id",
"$$productId"
]
}
}
},
{
"$unwind": "$product_items"
},
{
"$match": {
"$expr": {
"$eq": [
"$product_items._id",
"$$productItemId"
]
}
}
},
{
"$project": {
"_id": true,
"product": {
"_id": "$_id",
"product_name": "$product_name"
},
"product_purchase_quantity": "$$purchaseQuantity",
"product_item": "$product_items"
}
}
],
"as": "productResolved"
}
},
{
"$unwind": "$productResolved"
},
{
"$group": {
"_id": "$_id",
"add_to_cart_products": {
"$push": "$productResolved"
}
}
}
])