userSchema.methods function is not getting called in mongoose

I have written userSchema.methods.generateAuthToken() function as follows

UserSchema.methods.generateAuthToken = async function()
{
    const token = jwt.sign({_id: this._id.toString()}, "ThisIsSecret");
    this.tokens = this.tokens.concat({token});
    await this.save();
    return token;
}

I am calling this function in my code while authenticating with google. following is the google authentication code

passport.use(new GoogleStrategy({
    clientID: CLIENT_ID,
    clientSecret: CLIENT_SECRET,
    callbackURL: "http://localhost:8000/auth/google/notepad"
  },
  async function(accessToken, refreshToken, profile, email, cb) {
    try
    {
      var user = await User.find({googleId: email.id});
      if(user.length === 0)
      {
        const user = new User({googleId: email.id, fullName: email.displayName, email: email.emails[0].value});
        await user.save();
      }
      return cb(undefined, user);
    }
    catch(err)
    {
      return cb(err, undefined);
    }
  }
));

router.get('/auth/google', passport.authenticate('google',{scope: ['profile','email']}));
router.get('/auth/google/notepad', 
  passport.authenticate('google', { failureRedirect: '/' }),
  async function(req, res) {
    try
    {
      const token = await req.user.generateAuthToken();
      res.cookie('authToken', token);
    }
    catch(err)
    {
      console.log(err);
    }
    res.redirect('/')
  });
router.get('/',(req,res)=>
{
    res.send();
})

I am not going to use findOrCreate() method for some reason. instead I am checking whether a user already exists or not by using find() method of mongoose. The problem is, when a user does not exists, req.user.generateAuthToken method gets called but when a user already exists then it says generateAuthToken method does not exist. It should run every time.

You should added this section of code so that passport saves your user info.

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

Also in the main route don't forget to add the following lines if you are using express

app.use(passport.initialize());

In a Connect or Express-based application, passport.initialize() middleware is required to initialize Passport. If your application uses persistent login sessions, passport.session() middleware must also be used.

check the link for more info.