Separate database for each user using mongodb and mongoose

I am using mongodb and mongoose to handle my database needs. I currently have this:

mongoose.connect(secrets.db, {server:{poolSize: 1}});
mongoose.connection.on('error', function() {
  console.error('MongoDB Connection Error. Make sure MongoDB is running.');
});

As my mongo connection. I want to change that so that each user that logs in (using passport) gets their own database. So when I do show dbs I would like to see user_ObjectID_database_name how can I modify my code to get this result? I want it because I believe it is more secure from my research. I can not however find any good guides on how to do this. I am sure they exist but I have spent 4 ish hours looking with no luck on even a semi-okay one that I could guess at.

Currents I just have a users collection.

This is my current code that runs when a user hit /signup as a POST request.

exports.postSignup = function(req, res, next){
  req.assert('email', 'Please sign up with a valid email.').isEmail();
  req.assert('password', 'Password must be at least 6 characters long').len(6);

  var errors = req.validationErrors();

  if (errors) {
    req.flash('errors', errors);
    req.flash('form', {
      email: req.body.email
    });
    return res.redirect('/signup');
  }
  // calls next middleware to authenticate with passport
  passport.authenticate('signup', {
    successRedirect: '/dashboard', // Select redirect for post signup
    failureRedirect: '/signup',
    failureFlash : true
  })(req, res, next);
    next();
};

I call this as a middleware in my route to keep them cleaner.

EDIT **

Would sharding be a better solution for performance? I know it requires a lot of set up and planning but it might be worth it? Only question is security...