使用模型创建实例显示'不是函数'(节点使用猫鼬）

我创建了一个用户模型，并希望在users.js路由文件中使用它。但这表明userModel不是一个函数。有人知道这里发生了什么吗？感谢

//models/User.js 中的我的用户模型

var mongoose = require('mongoose');
var bcrypt = require('bcrypt-nodejs');
var url = 'mongodb://fr:fran98@ds013250.mlab.com:13250/node';
mongoose.connect(url); 
var db = mongoose.connection;

db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', function() {//when connected
        console.log('connected to mongo db');
        //User Schema
        var UserSchema = mongoose.Schema({
            username: {
                type:String,
                index:true
            },
            email: {
                type:String
            },
            password: {
                type:String,
                required:true,
                bcrypt:true
            },
            profileimage: {
                type:String
            }
        }, { timestamps: 'created_at' } );   
        //add "speak" functionality to our documents:  
        UserSchema.methods.speak = function(){
            var greeting = this.name
                ? "His name is " + this.name
                : "I do not have a nanme";
            console.log(greeting);
        }       
        module.exports = {
            userModel: mongoose.model('User',UserSchema),
            createUser: function(newUser,callback){
                bcrypt.hash(newUser.password, null, null, function(err, hash) {
                    // Store hash in your password DB.
                    newUser.password = hash;
                    newUser.save(callback);
                });
            }
        };               
});

//路由/用户.js

    const User = require('../modules/user');
    var userModel = User.userModel;
    var newUser = new userModel({
        username:username,
        email:email,
        password:password,
        profileimage: profileImageName
    });
    //Create User
    User.createUser(newUser,function(err,user){
        if(err){throw err}
        console.log(user);
    });

地址：//更多关于路由/用户.js

"use strict";
const express = require('express');
const router = express.Router();
const multer = require('multer');
const upload = multer({ dest: '/uploads' });
const User = require('../modules/user');
//this User returns: 1,User = mongoose.model('User',UserSchema)
//2: User.createUser = function(newUser,callback){
        //  newUser.save(callback);
        // }    
/* GET users listing. */
router.get('/', function(req, res, next) {
  res.send('respond with a resource');
});
router.get('/register', function(req, res, next) {
  res.render('register',{
      'title': 'Register',
      'errors':[]
  });
});
router.post('/register',upload.single('profileimage'), function(req, res, next) {
    var username = req.body.username;
    var email = req.body.email;
    var password = req.body.password;
    var password2 = req.body.password2;
    console.log(username,email,password,password2);
    //check for image field
    if(req.file){
        console.log('uploading file...');
        //file info
        var profileImageOriginalName = req.file.originalname;//the name before uploaded
        var profileImageName = req.file.filename;//name the server give it when it uploaded
        var profileImageMime = req.file.mimetype;
        var profileImagePath = req.file.path;
        var profileImageSize = req.file.size;
    }else{
        //set a default image
        profileImageName = 'noimage.png'; 
    }
    //form validation, using express-validator
req.checkBody({
 'email': {
    notEmpty: true,
    isEmail: {
      errorMessage: 'Invalid Email'
    }
  },
  'password': {
    notEmpty: true,
    isLength: {
      options: [{ min: 4, max:20 }],
      errorMessage: 'Password Must be between 4 and 8 chars long' // Error message for the validator, takes precedent over parameter message
    },
    errorMessage: 'Invalid Password' // Error message for the parameter
  },
  'password2': {
    notEmpty: true,
    matches: {
      options: [password, 'g'] // pass options to the validator with the options property as an array
      // options: [/example/i] // matches also accepts the full expression in the first parameter
    },
    errorMessage: 'Password Not Match' // Error message for the parameter
  },
  'username': { //
    //optional: true, // won't validate if field is empty
    notEmpty: true,
    isLength: {
      options: [{ min: 4, max: 50 }],
      errorMessage: 'Username Must be between 4 and 8 chars long' // Error message for the validator, takes precedent over parameter message
    },    
    errorMessage: 'Invalid Name'
  }
});

    // req.checkBody('username','userName field is required').notEmpty();
    // req.checkBody('email','Email field not valid').isEmail();
    // req.checkBody('password','Password field is required').notEmpty();
    // req.checkBody('password2','Password do not match').equals(req.body.password);
    //check for errors
    var errors = req.validationErrors();
    //or let errors = req.asyncValidationErrors();
    //errors is like below:
    //[
    //   {param: "email", msg: "required", value: "<received input>"},
    //   {param: "email", msg: "valid email required", value: "<received input>"},
    //   {param: "password", msg: "6 to 20 characters required", value: "<received input>"}
    // ]
    if(errors){
        res.render('register',{
            'title': 'Register',
            'errors': errors,
            'username':username,
            'email':email,
            'password':password,
            'password2':password2
        });
    }else{
        console.log('pass the validation.');
        var userModel = User.userModel;
        // var newUser = new userModel({
        //     username:username,
        //     email:email,
        //     password:password,
        //     profileimage: profileImageName
        // });
userModel.create({ 
            username:username,
            email:email,
            password:password,
            profileimage: profileImageName
}, function (err, res) {
  if (err) return handleError(err);
  console.log(res);
  // saved!
})

        //Create User
        User.createUser(newUser,function(err,user){
            if(err){throw err}
            console.log(user);
        });//user model
        // newUser.save(function(err,newUser){
        //     if(err){console.log(err);}
        //     console.log(newUser);
        // });
        //success Message
        req.flash('success','you are now registered and may log in');
        //redirect to the home page
        res.location('/');
        res.redirect('/');
    }
});


router.get('/login', function(req, res, next) {
  res.render('login',{
      'title': 'Login'
  });
});
module.exports = router;