FCC Advanced Node and Express Checkpoint 3

server.js

'use strict';

const express     = require('express');
const bodyParser  = require('body-parser');
const fccTesting  = require('./freeCodeCamp/fcctesting.js');
const session     = require('express-session');
const passport    = require('passport');
const mongo       = require('mongodb').MongoClient;
const ObjectID    = require('mongodb').ObjectID;
const LocalStrategy = require('passport-local');

const app = express();

fccTesting(app); //For FCC testing purposes
app.use('/public', express.static(process.cwd() + '/public'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));

app.set('view engine', 'pug')

app.use(session({
  secret: process.env.SESSION_SECRET,
  resave: true,
  saveUninitialized: true,
}));
app.use(passport.initialize());
app.use(passport.session());

mongo.connect(process.env.DATABASE, (err, db) => {
    if(err) {
        console.log('Database error: ' + err);
    } else {
        console.log('Successful database connection');

        passport.serializeUser((user, done) => {
          done(null, user._id);
        });

        passport.deserializeUser( (id, done) => {
            db.collection('users').findOne(
                {_id: new ObjectID(id)},
                (err, doc) => {
                    done(null, doc);
                }
            );
        });
      
        passport.use(new LocalStrategy(
          function(username, password, done) {
            db.collection('users').findOne({ username: username }, function (err, user) {
              console.log('User '+ username +' attempted to log in.');
              if (err) { return done(err); }
              if (!user) { return done(null, false); }
              if (password !== user.password) { return done(null, false); }
              return done(null, user);
            });
          }
        ));

        app.route('/')
          .get((req, res) => {
            res.render(process.cwd() + '/views/pug/index', {title: 'Hello', message: 'login', showLogin: true});
          });
      
        app.route('/login')
          .post(passport.authenticate('local', { failureRedirect: '/' }),(req,res) => {
               res.redirect('/profile');
          });
      
        app.route('/profile')
          .get((req,res) => {
               res.render(process.cwd() + '/views/pug/profile');
          });

        app.listen(process.env.PORT || 3000, () => {
          console.log("Listening on port " + process.env.PORT);
        });  
}});

I do not understand why FCC took the approach of using mongodb instead mongoose.

There is a major problem with mongoose or this was a personal choice of @JosephLivengood ?