How query and response for API NodeJS - node.js

Question

I want to query and response for API. I have 2 collections "Account" and "User". "User" has "id" field and "Account" has "user_id" field - both of us is one. One "User" has one or many Account and it has "id" field in "Account" collection. I want to query "id" from "User" collection and expect the result detail of this User and 1 field "accounts_id" into it.
This is all my expected result to look like:
https://sample-accounts-api.herokuapp.com/
I tried to collect "accounts_id" but I can not know how to query and add it to "User" detail.
This is my collect "accounts_id":
exports.findAllAccountIDforUser = (req, res) => {
Account
.find({user_id: req.param('user_id')})
.then(result => {
let a = result.map(data => {
return data.id;
})
res.send({'accounts_id':a});
})
}
My "User" Schema:
var mongoose = require('mongoose');
mongoose.Promise = require('bluebird');
var Schema = mongoose.Schema,
ObjectId = Schema.ObjectId;
const Account = require('../model/account');
var UserSchema = new mongoose.Schema({
id: {type: Number, unique: true},
name: String,
});
const User = mongoose.model('User', UserSchema);
module.exports = User;
My "Account" Schema:
var mongoose = require('mongoose');
mongoose.Promise = require('bluebird');
var Schema = mongoose.Schema,
ObjectId = Schema.ObjectId;
var AccountSchema = new mongoose.Schema({
id: {type: Number},
user_id: {type: Number, ref: 'User'},
name: String,
balance: Number
});
const Account = mongoose.model('Account', AccountSchema);
module.exports = Account;

Answer 1

using async-await you can do it in 4 lines.
get user
get all accounts related to that user
assign user.account_ids=accounts
send response.
exports.findAllAccountIDforUser = async(req, res) => {
let user=await User.findOne({id:req.param('user_id')});
let accounts= await Account.find({user_id: req.param('user_id')}).select(' _id id);
user.account_ids=accounts;
res.send(user);
}
another way is using promises like you are already doing it.
exports.findAllAccountIDforUser = (req, res) => {
User
.findOne({id:req.param('user_id')})
.then(user=>{
Account
.find({user_id: req.param('user_id')})
.then(result => {
let a = result.map(data => {
return data.id;
})
user.account_ids=a;
res.send(user);
})
})
}

Answer 2

I have answer for thís:
exports.findAlldetail = (req, res) => {
User
.aggregate([
{
$match: {
id: +req.param('id')}
},
{
$lookup: {
from: "accounts",
localField: "id",
foreignField: "user_id",
as: "account_ids"
}
},
{
$project: {
_id: 0,
id: 1,
name: 1,
"account_ids.id": 1
}
}
])
.then(result => {
result.account_ids = result.map(data => {
data.account_ids = data.account_ids.map(b => b.id);
})
res.send({"attributes":result});
})
}