passport serializeUser
passport.serializeUser(function(user, done) {
done(null, user.id);
}); │
│
│
└─────────────────┬──→ saved to session
│ req.session.passport.user = {id: '..'}
│
↓
passport.deserializeUser(function(id, done) {
┌───────────────┘
│
↓
User.findById(id, function(err, user) {
done(err, user);
}); └──────────────→ user object attaches to the request as req.user
});