Data not persisting/saving into MongoDB database - node.js

Question

I'm trying to figure out why my data hasn't been saving in MongoDB, despite calling the .save() method. I'm aware that this is an asynchronous process, but as you can see below, I set up a callback using .then() - which executes without error. The target database has a single empty collection in it, so I was expecting the data to appear there or in a new collection, neither of which happened. Here's the code for my index.js file:
const express = require('express');
const mongoose = require('mongoose');
const { body, validationResult } = require('express-validator/check');
const router = express.Router();
const Registration = mongoose.model('Registration');
router.get('/', (req, res) => {
res.render('form', { title: 'Registration Form'});
});
router.post('/',
[body('name')
.isLength(1)
.withMessage('Please enter a name.'),
body('email')
.isLength(1)
.withMessage('Please enter an email address.')
],
(req, res) => {
//console.log(req.body);
const errors = validationResult(req);
if(errors.isEmpty)
{
const registration = new Registration(req.body);
//registration.markModified('collection_1');
registration.save()
.then(() => { res.send('Thank you for your registration!'); })
.catch(() => { res.send('Sorry! Something went wrong.'); });
} else {
// re-renders form with error message
res.render('form', {
title: 'Registration form',
errors: errors.array(),
data: req.body,
});
}
});
module.exports = router;
Let me know if any other files would be useful. Any assistance would be greatly appreciated.
EDIT: Here's my start.js file.
require('dotenv').config();
const mongoose = require('mongoose');
require('./models/Registration');
mongoose.connect(process.env.DATABASE, { useMongoClient: true });
mongoose.Promise = global.Promise;
mongoose.connection
.on('connected', () => {
console.log(`Mongoose connection open on ${process.env.DATABASE}`);
})
.on('error', (err) => {
console.log(`Connection error: ${err.message}`);
});
const app = require('./app');
const server = app.listen(3000, () => {
console.log(`Express is running on port ${server.address().port}`);
});