Local and Google strategy in passport.js: issue when serializing user

问题

I've been trying to understand why I couldn't keep an user logged in once authenticated even though authentication itself was working. I even posted a question here: Passport.js - Local strategy doesn't authenticate

By trying to fix the issue, I finally worked out what's wrong.

The issue is the following: I have two different passport strategy, so I am serializing and deserializing the user twice. If I serialize the user with the local strategy first, local strategy will work, but Google's won't. And vice versa.

I put a comment to highlight the problem in app.js.

Here's the files:

app.js

const   express           = require("express"),
        mongoose          = require("mongoose"),
        bodyParser        = require("body-parser"),
        cookieSession     = require("cookie-session"),
        localStrategy     = require("passport-local"),
        passport          = require("passport");

const LocalUser = require("./models/localuser");

const keys = require("./config/keys"); // requiring keys

const authRoutes = require("./routes/auth"); // requiring auth routes
const mainRoutes = require("./routes/main");

//Initialize express app
const app = express();

mongoose.connect("mongodb://localhost/thoughtApp"); // connectiong database

app.use(express.static(__dirname + "/public"));
app.set("view engine", "ejs"); 
app.use(bodyParser.urlencoded({extended: true}));

app.use(cookieSession({
    maxAge: 24 * 60 * 60 * 1000,
    keys: [keys.session.cookieKey]
}));

//initialize passport 
app.use(passport.initialize());
app.use(passport.session());

passport.use(new localStrategy(LocalUser.authenticate()));
passport.serializeUser(LocalUser.serializeUser());
passport.deserializeUser(LocalUser.deserializeUser());

app.use(function(req, res, next){
    res.locals.user = req.user;
    next();
});

app.use("/", mainRoutes); //main routes
app.use("/auth", authRoutes); // setup auth routes

const passportSetup = require("./config/passport-setup"); /// THIS IS THE ISSUE

// IF BeFORE LINE 33 ( passport.use(new localStrategy(LocalUser.authenticate()));, GOOGLE LOGIN WORKS BUT LOCAL DOESNT; IF AFTER, LOCAL WORKS BUT GOOGE DOESN'T; PROBABLY DUE TO SERIALIZE AND DESARIALIZE BEING USED ALREADY

app.listen(process.env.PORT || 3000, () => {
    console.log("Server started.")
});

auth.js (auth routes)

const router = require("express").Router();
const passport = require("passport");

const LocalUser = require("../models/localuser");

const authCheck = function (req, res, next) {
    if (!req.user) {
        next();
    } else {
        res.redirect("/");
    }
};
//login
router.get("/login", authCheck, (req, res) => {
    res.render("login", {user: req.user});
});

router.post("/login", passport.authenticate("local", {
    successRedirect: "/",
    failureRedirect: "/login"
}), (req, res) => {

})
// logout
router.get("/logout", (req, res) => {
    //handle with passport
    req.logout();
    res.redirect("/");
});

//register
router.get("/signup", authCheck, (req, res) => {
    res.render("signup", {user: req.user});
});

router.post("/signup", (req, res) => {
    LocalUser.register(new LocalUser({username: req.body.username}), req.body.password, (err, user) => {
        if (err) {
            console.log(err);
            res.redirect("/auth/signup")
        }  
        passport.authenticate("local")(req, res, () => {
            console.log(user)
            res.redirect("/");
        })
    })

})
// google auth

router.get("/google", authCheck, passport.authenticate("google", {
    scope: ["profile"]
})) 
    //goes to google consent screen

    // callback for google to redirect to
router.get("/google/redirect", passport.authenticate("google"), (req, res) => {
    res.redirect("/profile");
});

module.exports = router;

passport-setup.js (google strategy setup)

const passport = require("passport");
const GoogleStrategy = require("passport-google-oauth20");
const keys = require("./keys");

const User = require("../models/user")

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

passport.deserializeUser((id, done) => {
    User.findById(id).then((user) => {
        done(null, user);
    });
});

passport.use(new GoogleStrategy({
    //options for the google strategy
    callbackURL: "/auth/google/redirect",
    clientID : keys.google.clientID,
    clientSecret : keys.google.clientSecret

    }, (accessToken, refreshToken, profile, done) => {
    //passport callback function
    // check if user exists already
    User.findOne({googleID: profile.id}).then((currentUser) => {
        if (currentUser) {
            console.log("user is: " + currentUser);
            done(null, currentUser);
        } else {
            new User({
                username: profile.displayName,
                googleID: profile.id
            }).save().then((newUser) => {
                console.log("new user created: " + newUser);
                done(null, newUser);
            })
        }
    })

    })
)

localuser.js

const mongoose = require("mongoose");
const passportLocalMongoose = require("passport-local-mongoose");

const localUserSchema = new mongoose.Schema({
    username: String,
    password: String
});

localUserSchema.plugin(passportLocalMongoose);

module.exports = mongoose.model("localUser", localUserSchema);

How can I solve this?