comparePassword issue

account.js

var u = require('./user')
, mongoose = require('mongoose')
, db = mongoose.createConnection('localhost', 'mydb');

var User = u.User;
var user = new User({username: 'test4', email:'test4@test.com', password: 'secret'});
user.save(function(err) {
  User.find({username: 'test4' }, function(err, user){
    if ( err ) console.log(err)
    user.comparePassword('hidden', function(err, isMatch) {
      console.log("isMatch", isMatch);
    });
  });
});

user.js

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

var UserSchema = new Schema({
  username    : String
  , email     : String
  , password  : String
});

UserSchema.methods.comparePassword = function(candidatePassword, cb) {
};

module.exports.User = db.model('User', UserSchema);