一、mongodb的使用：

1、聚合一般格式：

db.orders.aggregate([
   {
      $lookup: {
         from: "items",
         localField: "item",    // field in the orders collection
         foreignField: "item",  // field in the items collection
         as: "fromItems"
      }
   },
   {
      $replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$fromItems", 0 ] }, "$$ROOT" ] } }
   },
   { $project: { fromItems: 0 } }
])

2、数据样例：

db.books.drop();
db.books.insert({_id:1,name:'book1',author:[1,2]});
db.books.insert({_id:2,name:'book2',author:1});
db.books.insert({_id:3,name:'book3',author:2});
db.books.insert({_id:4,name:'book4'});

db.authors.drop();
db.authors.insert({_id:1,name:'john',nationality:'cn'});
db.authors.insert({_id:2,name:'Mark',nationality:'us'});
db.authors.insert({_id:3,name:'Shakespeare',nationality:'un'})

3、查出所有的《作者及作者的图书们》（不包括作者的_id,图书_id,图书的作者_id）

db.authors.aggregate([{$lookup:{from:'books',localField:'_id',foreignField:'author',as:'books'}},{$project:{_id:0,'books._id':0,'books.author':0}}]);

结果：

{ "name" : "john", "nationality" : "cn", "books" : [ { "name" : "book1" }, { "name" : "book2" } ] }
{ "name" : "Mark", "nationality" : "us", "books" : [ { "name" : "book1" }, { "name" : "book3" } ] }
{ "name" : "Shakespeare", "nationality" : "un", "books" : [ ] }

4、查出所有的 《图书及图书的作者们》，不包括：图书的_id,作者的_id

db.books.aggregate([{$lookup:{from:'authors',localField:'author',foreignField:'_id',as:'author'}},{$project:{_id:0,'author._id':0}}]);

结果：

{ "name" : "book1", "author" : [ { "name" : "john", "nationality" : "cn" }, { "name" : "Mark", "nationality" : "us" } ] }
{ "name" : "book2", "author" : [ { "name" : "john", "nationality" : "cn" } ] }
{ "name" : "book3", "author" : [ { "name" : "Mark", "nationality" : "us" } ] }
{ "name" : "book4", "author" : [ ] }

二、mongoose的populate使用：

根据【一.2】的数据，我们编写下代码，进行关联查询

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

mongoose.connect('mongodb://localhost/test2',{ useNewUrlParser: true,useUnifiedTopology: true })

const bookSchema=Schema({
    _id:Number,
    name:String,
    author:[{type:Number,ref:'Author'}]
})

const authorSchema=Schema({
    _id:Number,
    name:String,
    nationality:String
})
const Author=mongoose.model('Author',authorSchema)
const Book=mongoose.model('Book',bookSchema)


Book.find({})
     .populate('author')
    .exec()
    .then(books=>{
        console.log(books)
    })
    .catch(err=>console.log(err))
    .finally(()=>mongoose.disconnect())