mongoose move ref document to main document as DTO - node.js

Question

I wanna to move sub document to main documents, and return single DTO without any nested document, below is my sample data.js
data.js
const mongoose = require('mongoose');
//city
const citySchema = new mongoose.Schema({
cityName: { type: String, required: true, unique: true },
});
const City = mongoose.model('City', citySchema);
//country
const countrySchema = new mongoose.Schema({
countryName: { type: String, required: true, unique: true },
});
const Country = mongoose.model('Country', countrySchema);
//user
const userSchema = new mongoose.Schema({
username: { type: String, required: true },
city: {
type: mongoose.Schema.Types.ObjectId,
ref: 'City',
required: true,
},
country: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Country',
required: true,
},
createdAt: { type: Date, required: true },
updatedAt: { type: Date, required: true },
});
const User = mongoose.model('User', userSchema);
function getUser(id) {
return User.findById(id)
.populate('city')
.populate('country')
.exec();
};
Current return JSON Response for User:
{
"_id": "6321ac3d14a57c2716f7f4a0",
"name": "David",
"city": {
"_id": "63218ce557336b03540c9ce9",
"cityName": "New York",
"__v": 0
},
"country": {
"_id": "632185bbe499d5505cafdcbc",
"countryName": "USA",
"__v": 0
},
"createdAt": "2022-09-14T10:26:05.000Z",
"__v": 0
}
How do I move the cityName and countryName to main model, and response JSON as below format?
{
"_id": "6321ac3d14a57c2716f7f4a0",
"username": "David",
"cityName": "New York",
"countryName": "USA",
"createdAt": "2022-09-14T10:26:05.000Z",
}

Answer 1

Using aggregation you can try something like this:
db.user.aggregate([
{
"$match": {
_id: "6321ac3d14a57c2716f7f4a0"
}
},
{
"$lookup": {
"from": "city",
"localField": "city",
"foreignField": "_id",
"as": "city"
}
},
{
"$lookup": {
"from": "country",
"localField": "country",
"foreignField": "_id",
"as": "country"
}
},
{
"$addFields": {
"country": {
"$arrayElemAt": [
"$country",
0
]
},
"city": {
"$arrayElemAt": [
"$city",
0
]
}
}
},
{
"$addFields": {
"countryName": "$country.countryName",
"cityName": "$city.cityName"
}
},
{
"$unset": [
"country",
"city"
]
}
])
Here's the playground link.
The other probably simpler way of doing this would be, modify your function like this:
function getUser(id) {
const user = User.findById(id)
.populate('city')
.populate('country')
.exec();
if(user.country) {
user.countryName = user.country.countryName;
}
if(user.city) {
user.cityName = user.city.cityName;
}
delete user.country;
delete user.city;
return user
};