How do you implement a mongo aggregate pipeline within the pre middleware? - node.js

Question

I have two schemas - Products & Orders - the each order contains an array of products referenced by the object.id. What I'm trying to do is $sum over the product.sellPrice in the array and set totalSale before saving the new order document.
const orderSchema = new mongoose.Schema({
products: [
{
type: mongoose.Schema.Types.ObjectId,
ref: "Product"
}
],
totalSale: {
type: Number,
min: 0
}
});
const productSchema = new mongoose.Schema({
name: {
type: String,
unique: true,
required: true
},
sellPrice: {
type: Number,
min: 0,
required: true
}}):
So each order would contain an array of products. So what tried doing was in the orderSchema.pre('save') hook, I tried running an aggregation like so:
orderSchema.pre("save", function(next) {
let order = this;
order.totalSale = order.aggregate([
{ $unwind: { path: "$products" } },
{ $group: { _id: null, $sum: {} } }
]);
Is this even possible or is there a better way to sum up the prices of each product for the order. It doesnt really have to be pre('save')

Answer 1

First of all, yes, it is possible to use aggregate function in pre.
If you want to use it, you need to add $lookup in your aggregation pipeline to populate product data into the order.
How about just using find() on Product?
const Product = mongoose.model('Product');
OrderSchema.pre('save', function (next) {
let order = this;
// if products array is empty, skip the process
if (!this.products || !Array.isArray(this.products) || !this.products.length < 1) {
return next();
}
Product.find({ $in: this.products }, { sellPrice: 1 })
.lean()
.exec(function (err, products) {
let totalSale = 0;
if (Array.isArray(products)) {
products.forEach(product => {
totalSale += product.sellPrice;
});
}
order.totalSale = totalSale;
next(err);
});
});

Answer 2

orderSchema.pre("save", function(next) {
let order = this;
let temp = order.products.map(product => {
return { _id: mongoose.Types.ObjectId(product) };
});
let totalSale = 0;
let promises = Product.find({ _id: { $in: temp } }).exec();
promises
.then(data => {
data.forEach(product => {
totalSale += product.sellPrice;
});
order.totalSale = totalSale;
next();
})
.catch(err => {
console.error(err);
});
});
Since find returns all matching the conditions, I saved them in a variable and then iterated over them.