Populate Booking con mongoose

Booking`

const express = require('express');
const mongoose = require('mongoose');

// const Apartment = require('./modelos/apartments').Apartment;
const Bookings = require('./modelos/bookings').Bookings;
// const Users = require('./modelos/bookings').Users;

const app = express();

//mongoose.set(`debug`, true);

// MONGODB DATABASE ACCESS
const MONGODB_URI =
  //'mongodb+srv://root:BIT-MongoDB@cluster0-prpps.mongodb.net/bookingApartments';

  'mongodb+srv://friendlyrent:8VnQsLJikYPC0C4G@cluster0.7mjts.mongodb.net/bookingsApartments';

mongoose
  .connect(MONGODB_URI, { useNewUrlParser: true, useUnifiedTopology: true })
  .then(result => {
    Bookings.findOne()
    .populate('idUser')
    .populate('idApartment')
    .then( apartment => {
        console.log('The code service is:', apartment);
      });
  })
  .catch(err => {
    console.log('Error al conectar a la base de datos:', err);
  });