express - Node.js连接MongoDB后，数据无法读取

使用 Node.js + express + MongoDB 搭建RESTful API服务。

现在遇到的问题是：数据库连接成功后，无法读取集合Collection内容。

app.js

var express = require('express');
var mongoose = require('mongoose');
var db = mongoose.connect('mongodb://localhost/db/workflow');
db.connection.on("error", function (error) {
    console.log("数据库连接失败：" + error); 
});
db.connection.on("open", function () {
    console.log("------数据库连接成功！------");
});
var bodyParser = require('body-parser');
var app = express();

// uncomment after placing your favicon in /public
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(require('node-compass')({mode: 'expanded'}));
app.use(express.static(path.join(__dirname, 'public')));

require('./models/group_model');
require('./routes/group')(app);

// 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 handler
app.use(function(err, req, res, next) {
  // set locals, only providing error in development
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  // render the error page
  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

models/group_model.js

var mongoose = require('mongoose'),
    Schema = mongoose.Schema;
var groupSchema = new Schema({
    name:'string'
});

module.exports = mongoose.model("group",groupSchema);

routes/group.js

var express = require('express');
module.exports = function (app) {
  var group = require('../controllers/group_controller');
  app.get('/group/get',group.getGroup);
};

controllers/group_controller.js

var mongoose = require('mongoose');
var group = mongoose.model('group');
exports.getGroup = function (req, res) {
    group.find({}).exec(function (err, data) {
        if(!data){
            res.json({code: 404, msg: "Empty"});
        }else{
            res.json({code: 0, msg: "success", data: data, count: data.length});
        }
    });
};

访问 http://localhost:3000/group/get 返回结果：

{
    code: 0,
    msg: "success",
    data: [ ],
    count: 0
}

正常的结果应该是：data中存在数据。

请各路高手赐教...