Node.js 如何获取在另一个模型中定义的mongoose 数据库的架构

这是我的文件夹 struct :

+-- express_example
|---- app.js
|---- models
|-------- songs.js
|-------- albums.js
|---- and another files of expressjs

我在文件歌曲中的代码.js

var mongoose = require('mongoose')
, Schema = mongoose.Schema
, ObjectId = Schema.ObjectId;

var SongSchema = new Schema({
name: {type: String, default: 'songname'}
, link: {type: String, default: './data/train.mp3'}
, date: {type: Date, default: Date.now()}
, position: {type: Number, default: 0}
, weekOnChart: {type: Number, default: 0}
, listend: {type: Number, default: 0}
});

module.exports = mongoose.model('Song', SongSchema);

这是我在文件相册中的代码.js

var mongoose = require('mongoose')
, Schema = mongoose.Schema
, ObjectId = Schema.ObjectId;

var AlbumSchema = new Schema({
name: {type: String, default: 'songname'}
, thumbnail: {type:String, default: './images/U1.jpg'}
, date: {type: Date, default: Date.now()}
, songs: [SongSchema]
});

module.exports = mongoose.model('Album', AlbumSchema);

How can I make albums.js know SongSchema to be defined AlbumSchema