引用mongoose模式的类型出错

我正在使用mongoose ODM开发一个使用mongodb的节点应用程序.我在类型引用驻留在不同文件中的模式时收到错误.

我在user.js文件中有以下代码:

var mongoose = require('mongoose');
var Trip = require('./trip');

var userSchema = mongoose.Schema({
    firstName: String,
    lastName: String,
    salt: String,
    hash: String,
    emailAddress: {
        type: String,
        unique: true
    },
    trips: [{Type: mongoose.Schema.Types.ObjectId, ref: 'Trip'}]
});

module.exports = mongoose.model('User', userSchema);

userSchema具有对tripSchema的类型引用.

我的trip.js文件中的代码是:

    var tripSchema = mongoose.Schema({
      name: String,
      location: String,
      arrivalDate: Date,
      departureDate: Date,
      type: String});

   module.exports = mongoose.model('Trip', tripSchema);

当我运行该应用程序时,我收到以下错误:

    /usr/lib/node_modules/mongoose/lib/schema.js:360
    throw new TypeError('Undefined type at `' + path +
          ^
TypeError: Undefined type at `trip.ref`
  Did you try nesting Schemas? You can only nest using refs or arrays.
    at Function.Schema.interpretAsType       (/usr/lib/node_modules/mongoose/lib/schema.js:360:11)
    at Schema.path (/usr/lib/node_modules/mongoose/lib/schema.js:303:29)
    at Schema.add (/usr/lib/node_modules/mongoose/lib/schema.js:217:12)
    at Schema.add (/usr/lib/node_modules/mongoose/lib/schema.js:212:14)
    at new Schema (/usr/lib/node_modules/mongoose/lib/schema.js:73:10)
    at Function.Schema.interpretAsType (/usr/lib/node_modules/mongoose/lib/schema.js:345:44)
    at Schema.path (/usr/lib/node_modules/mongoose/lib/schema.js:303:29)
    at Schema.add (/usr/lib/node_modules/mongoose/lib/schema.js:217:12)
    at new Schema (/usr/lib/node_modules/mongoose/lib/schema.js:73:10)
    at Mongoose.Schema (/usr/lib/node_modules/mongoose/lib/schema.js:53:12)

我无法弄清楚错误的原因.如果两个模式都在同一个文件中,则代码运行正常.但是当我在两个不同的文件中分离出模式时,我遇到了错误.我该如何解决这个错误？任何帮助将不胜感激.