Node.js and Passport Object has no method validPassword

Please note that I'm a total beginner in Node.js so please be patient with me :)

I'm using Node.js + Express 3 + Passport to create a simple authentication(local) just to play around

and what I've reached so far that when a wrong username or password entered user is redirected to an error page

but when the user enters a correct username and password I get this error

node_modules\mongoose\lib\utils.js:435
    throw err;
          ^
TypeError: Object { _id: 50b347decfd61ab9e9e6768f,
username: 'saleh',
password: '123456' } has no method 'validPassword'

I'm not sure what's wrong there

app.js (I removed the unnecessary code):

  var passport = require('passport')
  , LocalStrategy = require('passport-local').Strategy;

  app.configure(function(){
  app.set('port', process.env.PORT || 3000);
  app.set('views', __dirname + '/views');
  app.set('view engine', 'ejs');
  app.use(express.favicon());
  app.use(express.logger('dev'));
  app.use(express.bodyParser());
  app.use(express.methodOverride());
  app.use(passport.initialize());
  app.use(passport.session());
  app.use(app.router);
  app.use(express.static(path.join(__dirname, 'public')));
});



var mongoose = require('mongoose');
var db = mongoose.createConnection('localhost', 'authTest');

var authSchema = mongoose.Schema({ 
  username: 'string',
  password: 'string'
});

var User = db.model('users', authSchema);


passport.use(new LocalStrategy(
  function(username, password, done) {
    User.findOne({ username: username }, function (err, user) {
      if (err) { return done(err); }
      if (!user) {
        return done(null, false, { message: 'Incorrect username.' });
      }
      if (!user.validPassword(password)) {
        return done(null, false, { message: 'Incorrect password.' });
      }
      return done(null, user);
    });
  }
));



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

passport.deserializeUser(function(id, done) {
  User.findById(id, function(err, user) {
    done(err, user);
  });
});




app.post('/login',
  passport.authenticate('local', { successRedirect: '/',
                                   failureRedirect: '/login/error',

                                  })
);

and now in routes/login.js

var mongoose = require('mongoose');
var db = mongoose.createConnection('localhost', 'authTest');

var authSchema = mongoose.Schema({ 
    username: 'string',
    password: 'string'
});

var User = db.model('users', authSchema);

exports.index = function(req, res){
User.find(function (err, list) {
        res.render('login', { title: 'Usernames and Passwords', users: list,msg:""});
    });
};

Thanks for your time.