var mongoose = require('mongoose'), Schema = mongoose.Schema
 
var personSchema = Schema({
 // _id默认为Schema.Types.ObjectId类型
 _id  : Number,
 name : String
});

var storySchema = Schema({
 creator : { type: Number, ref: 'Person' },
 title : String,
 // 可以看到外键引用可以定义在嵌套的属性中。
 fans  : [{ type: Number, ref: 'Person' }]
});

var Story = mongoose.model('Story', storySchema);
var Person = mongoose.model('Person', personSchema);

var alice = new Person({ _id: 0, name: 'Alice'});
# 保存其ID即可
var story = new Story({ title: 'xx', creator: alice._id });

Story.findOne({title: 'yy'})
 .populate('creator')
 .exec(function(err, story){
  if(err) throw err;
  console.log(story.creator.name);
 });

Story.findOne({title: 'xx'})
 .populate('creator', 'name') // 只返回Person的name字段
 .exec(function(err, story){
  if(err) throw err;
  console.log(story.creator.name);
 });

Story.find(...)
 .populate('creator fans')
 .exec();
Story.find(...)
 .populate('creator')
 .populate('fans')
 .exec();

Story.find(...)
 .populate({
  path: 'fans',
  match: { age: { $gte: 21 }},
  select: 'name',
  options: { limit: 5 }
 });
 .exec();

var userSchema = new Schema({
 name: String,
 friends: [{ type: ObjectId, ref: 'User' }]
});
User.
 findOne({ name: 'Val' }).
 populate({
  path: 'friends',
  populate: { path: 'friends' }
 });

var userSchema = new Schema({
 _id: Number,
 name: String,
 teacher: Number
});
User.
 findOne({ name: 'Val' }).
 populate({
  path: 'teacher',
  model: 'User' // 在User集合中查找该ID
 })

var user = { name: 'Indiana Jones', weapon: 389 };
User.populate(user, { path: 'weapon', model: 'Weapon' }, function (err, users) {
 console.log(user.weapon.name);
});

var user = { name: 'Indiana Jones', weapon: 389 };
Weapon.populate(user, { path: 'weapon' }, function (err, users) {
 console.log(user.weapon.name);
});