Problems about mongoose, no warnings or errors but it don't work

guys. I plan to create a signup page for my blog. But there's something wrong with the mongoose, I can't find the errors at all.

At first, I create a schema in mongo.js,and exports a model as below:

var db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open',function(){
      console.log("success open db!");
    });

var Schema = mongoose.Schema;

//user schema
var userSchema = new Schema({
  name: {
    type: String
  },
  password: {
    type: String
  },
  avatar: {
    type: String
  },
  gender: {
    type: String,
    enum: ['m', 'f', 'x']
  },
  bio: {
    type: String
  }
});
//schema level index
userSchema.index({ name: 1 }, { unique: true });
//create user model
exports.User = mongoose.model('User', userSchema);

Then I call Model.create to save my data to the db,

var User = require('../lib/mongo').User;


module.exports = {
  //save data
  create: function(user) {
    return User.create(user);
  }
};

At last, I handle the promise which's from the model.create(),

var userModel = require('../models/users');
  //save date
  userModel.create(user)
    .then(function (result) {
      // 此 user 是插入 mongodb 后的值，包含 _id
      user = result.ops[0];
      // 将用户信息存入 session
      delete user.password;
      req.session.user = user;
      // 写入 flash
      req.flash('success', '注册成功');
      // 跳转到首页
      res.redirect('/posts');
    })
    .catch(function (e) {
      // 用户名被占用则跳回注册页，而不是错误页
      if (e.message.match('E11000 duplicate key')) {
        req.flash('error', '用户名已被占用');
        return res.redirect('/signup');
      }
      next(e);
    });

});

but it didn't work, when I fill the from and submit, No user model was created and the website freezes... Are there wrong operation of mongoose? Thanks.

via Benny201