[译]Mongoose指南 - Population

MongoDB没有join, 但是有的时候我们需要引用其它collection的documents, 这个时候就需要populate了.

我们可以populate单个document, 多个document, plain object, multiple plain objects或query返回的全部object.

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

var personSchema = new Schema({
    _id: Number,
     name: String,
     age: Number,
     stories: [{type: Schema.Types.ObjectId, ref: 'story'}]
});

var storySchema = new Schema({
     _creator: {type: Number, ref: 'Person'},
     title: String,
     fans: [{type: Number, ref: 'Person'}]
});

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

目前为止我们创建了两个model. Person model有一个stories字段是以个ObjectId的集合. ref是告诉mongoose哪个model应该被popluate.

Story有一个_creator字段是Number类型的他是personSchema的Id

Saving ref

保存一个ref

var arron = new Person({_id:0, name: 'Aaron', age: 100});

arron.save(function(err){
   if(err) return handleError(err);

   var story1 = new Story({
      titile: 'Once upon a timex',
      _creator: arron._id    //分配person的_id
   });

   story1.save(function(err){
      if(err) return handleError(err);
   });

});

Populate

填充story的_creator

Story
  .findOne({title: 'Once upon a timex'})
  .populate('_creator')
  .exec(fucntion(err, story){
      if(err) return handleError(err);
      console.log(story._creator.name) // Aaron
   });

Field selection

如果我们不想为story填充整个_creator 我们只想填充_creator的name怎么搞呢？

Story
.findOne({ title: /timex/i })
.populate('_creator', 'name') // only return the Persons name
.exec(function (err, story) {
  if (err) return handleError(err);

  console.log('The creator is %s', story._creator.name);
  // prints "The creator is Aaron"

  console.log('The creators age is %s', story._creator.age);
  // prints "The creators age is null'
})

一次填充多个paths

如果我们想一次填充多个path怎么过搞呢

Story
.find(...)
.populate('fans author') // space delimited path names
.exec()

另一种写法

Story
.find(...)
.populate('fans')
.populate('author')
.exec()

按条件填充

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