How to process the fetched data from mongoDB? - node.js

Question

I want my order_total to be calculated from results after joining two different collections
I'm building an Food Ordering App. I've seperate model for item:"ItemModel"
Here's my orderSchema which is being followed while ordering the items. I've referred to the ItemModel in case of Item_Id
My OrderSchema:
const cartOrderSchema = new mongoose.Schema({
items: [
{
item_id: { type: mongoose.Schema.Types.ObjectId, ref: "itemModel" },
quantity: { type: Number, default: 1 },
},
],
user_id: {
type: mongoose.Schema.Types.ObjectId,
ref: "userModel",
},
date: {
type: Date,
default: new Date(),
},
order_total: {
type: Number,
default: 0,
},
delivery_address: {
type: String,
default: "GGn",
},
payment_type: {
type:String,
default:"Cash"
},
});
I'm using this query:
await req.user.populate({
path: "mycartOrder",
populate:{
path:"items.item_id"
}
});
res.status(200).send(req.user.mycartOrder);
& getting result as:
{
"order_total": 0,
"delivery_address": "GGn",
"payment_type": "Cash",
"_id": "6270d84d89e3be5f291ee923",
"items": [
{
"item_id": {
"item_type": "Veg",
"_id": "62614934693ffd8d8f7979e9",
"name": "Cheese Burger",
"cuisine_category": "Starters",
"price": 67,
"description": "Aloo patty enclosed within Bun Tikki with melted cheese",
"store_id": "626148ca693ffd8d8f7979e2",
"__v": 0
},
"quantity": 10,
"_id": "6270d84d89e3be5f291ee924"
},
{
"item_id": {
"item_type": "Veg",
"_id": "62614977693ffd8d8f7979ec",
"name": "Paneer Lababdar",
"cuisine_category": "Indian",
"price": 249,
"description": "Panner vegetable cooked in very rich Gravy",
"store_id": "626148ca693ffd8d8f7979e2",
"__v": 0
},
"quantity": 3,
"_id": "6270d84d89e3be5f291ee925"
}
],
"date": "2022-05-03T07:22:03.907Z",
},
What I want?
I want that this order_total should be calculated automatically when the query is executed by multiplying the item's price with quantity.
Please HELP!!

Answer 1

If your collection is called cart (I'm not familiar with node.js or mongoose), you can use the $unwind operator and unwind your items. Then use the $addFields operator to add a new field overwriting your order_total, with the value to be the multiplication of the price and quantity. Then you'll have to group them back together, using the $sum operator on all the order_total fields, and then re-add all the previous fields..
db.cart.aggregate([
{
$unwind: "$items"
},
{
$addFields: {
"order_total": {
$multiply: [ "$items.quantity", "$items.item_id.price" ]
}
}
},
{
$group: {
_id: "$_id",
order_total: {$sum: "$order_total"},
delivery_address: { $first: "$delivery_address" },
payment_type: { $first: "$payment_type" },
items: { $push: "$items" },
date: {$first: "$date" }
}
}
])
You can skip the $addFields step and do the sum and multiply all in one step but I wanted to show both for clarity:
db.cart.aggregate([
{
$unwind: "$items"
},
{
$group: {
_id: "$_id",
order_total: {$sum: {$multiply: [ "$items.quantity", "$items.item_id.price" ]}},
delivery_address: { $first: "$delivery_address" },
payment_type: { $first: "$payment_type" },
items: { $push: "$items" },
date: {$first: "$date" }
}
}
])
You might need a match stage if you only want one specific cart.