Joining two mongoose collections with many to many relationship - node.js

Question

I have two models which are Product & Category with many to many relationship.
Following are my models
This is the model for Product
//Product.js
const mongoose = require("mongoose");
const { Schema } = mongoose;
const { ObjectId } = Schema;
const product = {
name: {
type: String,
required: true,
trim: true,
minlength: [2, "Too short"],
maxlength: [32, "Too long"],
unique: true,
},
slug: {
type: String,
unique: true,
lowercase: true,
index: true,
},
category: [
{
type: ObjectId,
ref: "Category",
},
],
description: {
type: String,
maxlength: 200,
},
price: {
type: Number,
required: true,
trim: true,
maxlength: 32,
},
shipping: {
type: String,
enum: ["Yes", "No"],
},
color: [
{
type: String,
enum: ["Black", "Brown", "Silver", "White", "Blue"],
},
],
sold: {
type: Number,
default: 0,
},
quantity: {
type: Number,
},
images: [
{
public_id: String,
url: String,
},
],
};
const productSchema = new Schema(product, { timestamps: true });
const Product = mongoose.model("Product", productSchema);
module.exports = Product;
This is the model for Category
//Category.js
const mongoose = require("mongoose");
const { Schema } = mongoose;
const { ObjectId } = Schema;
const category = {
name: {
type: String,
required: true,
trim: true,
max: 32,
unique: true,
},
subCategories: [
{
type: Schema.Types.ObjectId,
ref: "Category",
},
],
parent: {
type: Schema.Types.ObjectId,
ref: "Category",
},
products: [
{
type: ObjectId,
ref: "Product",
},
],
slug: {
type: String,
required: "URL can't be empty",
unique: true,
},
};
const categorySchema = new Schema(category, { timestamps: true });
//Validate the slug to ensure proper url is given
// categorySchema.path("slug").validate((val) => {
// urlRegex =
// /(ftp|http|https):\/\/(\w+:{0,1}\w*#)?(\S+)(:[0-9]+)?(\/|\/([\w#!:.?+=&%#!\-/]))?/;
// return urlRegex.test(val);
// }, "Invalid URL.");
const autoPopulateChildren = function (next) {
this.populate("subCategories");
// this.populate("parent");
next();
};
categorySchema
.pre("findOne", autoPopulateChildren)
.pre("findById", autoPopulateChildren)
.pre("find", autoPopulateChildren);
const Category = mongoose.model("Category", categorySchema);
module.exports = Category;
So, product has a foreign keys of category in array of category. Whereas Category has foreign keys of product in array of products. How do you join them ?
I tried this & it doesn't work. It returns empty object
router.get("/products", [RequireSignIn], async (req, res) => {
try {
const products = await Product.find({}).aggregate({
$lookup: {
from: "Category",
localField: "category",
foreignField: "_id",
as: "ProductCategories",
},
});
return res.status(200).send(products);
} catch (error) {
return res.status(200).send(error);
}
});