How to 'join' or populate an array - node.js

Question

Here is my very basic product schema:
const productSchema = new Schema({
productName: {
type: String,
required: true,
},
productDescription: {
type: String,
},
productPrice: {
type: Number,
},
});
module.exports = mongoose.model("Product", productSchema);
These products are listed out and a user can add a quantity for each. I am storing in an array of objects as per below. I want to join these two collections together so that I can output the qty of products selected by the user. I believe I need to use populate here but not sure how to set up the Refs and so on.
const PartySchema = new Schema({
firstName: {
type: String,
required: true,
},
lastName: {
type: String,
required: true,
},
catering: [{ id: mongoose.Types.ObjectId, qty: Number }],
created_at: {
type: Date,
default: Date.now,
},
});
module.exports = mongoose.model("Party", PartySchema);

Answer 1

I'm sharing this solution with the assumption that the catering field is the sub-document array pointing to the Product Schema:
The Product Schema is fine so it stays the same (although to keep to convention I would advice naming your schema 'Products' instead of 'Product', Mongo Naming Covention):
const productSchema = new Schema({
productName: {
type: String,
required: true,
},
productDescription: {
type: String,
},
productPrice: {
type: Number,
},
});
module.exports = mongoose.model("Products", productSchema);
And next the Party Schema would be:
const PartySchema = new Schema({
firstName: {
type: String,
required: true,
},
lastName: {
type: String,
required: true,
},
catering: [{
id: {
type: mongoose.Types.ObjectId,
ref: 'Products',
},
qty: {
type: Number,
}
}],
created_at: {
type: Date,
default: Date.now,
},
});
module.exports = mongoose.model("Parties", PartySchema);