mongodb - mongoskin 使用collection方法失败

在使用EXPRESS + NODEJS + MONGODB + MONGOSKIN来快速搭建web应用的时候，读取数据失败。
在打开数据库连接，创建了一条数据的前提下，我是这样做的：

1：在app.js中数据连接

// Database
var mongo = require('mongoskin');
var db = mongo.db("mongodb://localhost:27017/learn3", {native_parser:true});

2：让数据库连接到http请求上

// Make our db accessible to our routerapp.use(function(req,res,next){
req.db = db;
next();
});

贴出app.js的代码如下：

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var bodyParser = require('body-parser');
// Database
var mongo = require('mongoskin');
var db = mongo.db("mongodb://localhost:27017/learn3", {native_parser:true});

var routes = require('./routes/index');
var users = require('./routes/users');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

// uncomment after placing your favicon in /public
//app.use(favicon(__dirname + '/public/favicon.ico'));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', routes);
app.use('/users', users);

// Make our db accessible to our router
app.use(function(req,res,next){
    req.db = db;
    next();
});

// catch 404 and forward to error handler
app.use(function(req, res, next) {
    var err = new Error('Not Found');
    err.status = 404;
    next(err);
});

// error handlers

// development error handler
// will print stacktrace
if (app.get('env') === 'development') {
    app.use(function(err, req, res, next) {
        res.status(err.status || 500);
        res.render('error', {
            message: err.message,
            error: err
        });
    });
}

// production error handler
// no stacktraces leaked to user
app.use(function(err, req, res, next) {
    res.status(err.status || 500);
    res.render('error', {
        message: err.message,
        error: {}
    });
});


module.exports = app;

3：在.\routes\users.js 编辑如下：

var express = require('express');
var router = express.Router();

/**
 * GET userlist.
 */
router.get('/userlist', function(req, res) {
    var db = req.db;
    db.collection('userlist').find().toArray(function(err, items) {
        res.json(items);
    })
  // res.send('respond with a resource');
});

module.exports = router;

4：打开服务，npm start 后，访问http://localhost:3000/users/userlist，发现报错了。
错误提示是：Cannot call method 'collection' of undefined。
查看网络，发现这样的用法也是可以的，请各位路过的仁兄帮忙看看呗。