1、lookup简介

MongoDB作为一个nosql类型的数据库，并没有像关系型数据库那样有着非常强大的外键(表与表)关联，但是MongoDB 3.2中增加了 $lookup 这个很牛逼的操作符，能够让你实现多表关联查询，而且它被放到了 aggregate 这种轻量级的pipeline框架上，一个字：牛

Aggregation支持的操作符

假设我们在devops库中有一个hosts集合以及一个gameapp集合，我们的gameapp集合中的文档 host_id 字段关联到了 hosts 文档的 _id 字段。

2、模拟数据

// 进入到devops库
use devops
// 插入数据
db.hosts.insertMany([
	{host: 'hdss7-21', private_ip: '172.16.7.21', public_ip: '202.100.23.21', region_id: '杭州', status: true},
	{host: 'hdss7-22', private_ip: '172.16.7.22', public_ip: '202.100.23.22', region_id: '杭州', status: true},
db.gameapp.insertMany([
	{host_id:ObjectId('5ef070ee381fbaacea727429'), name: 'qysg10001', version: '37184', port: 80001},
	{host_id:ObjectId('5ef070ee381fbaacea727429'), name: 'qysg10002', version: '37184', port: 80002},
	{host_id:ObjectId('5ef070ee381fbaacea727429'), name: 'qysg10003', version: '37184', port: 80003},
	{host_id:ObjectId('5ef070ee381fbaacea72742a'), name: 'qysg10004', version: '37184', port: 80004},
	{host_id:ObjectId('5ef070ee381fbaacea72742a'), name: 'qysg10005', version: '37184', port: 80005},
	{host_id:ObjectId('5ef070ee381fbaacea72742a'), name: 'qysg10006', version: '37184', port: 80006},
	{host_id:ObjectId('5ef070ee381fbaacea72742a'), name: 'qysg10007', version: '37184', port: 80007},
// 验证数据是否正确
db.hosts.find()
db.gameapp.find()
3、查询操作
查询指定游戏服所关联的主机信息，最后只显示主机的host、private_ip和status信息
1）$lookup使用
db.gameapp.aggregate([
	$match: {name: 'qysg10001'} 
	$lookup: {
	 	from: 'hosts',
	 	localField: 'host_id',
	 	foreignField: '_id',
	 	as: 'host_info'
简单介绍$lookup中的参数：
form：需要关联的表（hosts）
localField：gameapp关联到hosts的键(字段)
foreignField：hosts被关联到gameapp的localField的键(字段)
as：对应的外键集合数据（可能存在一对多的情况）
返回的数据
	"_id": ObjectId("5ef0737e381fbaacea727432"), 
	"host_id": ObjectId("5ef070ee381fbaacea727429"), 
	"name": "qysg10001", 
	"version": "37184", 
	"port": 80001, 
	"host_info": [{   
	"_id": ObjectId("5ef070ee381fbaacea727429"),
	"host": "hdss7-21",   
	"private_ip": "172.16.7.21",   
	"public_ip": "202.100.23.21",   
	"region_id": "杭州",   
	"status": true 
2）可以看到host_info返回元素装在列表中，需要先把host_info外层列表除去，通过$unwind实现对host_info拆分
db.gameapp.aggregate([
	$match: {name: 'qysg10001'} 
	$lookup: {
	 	from: 'hosts',
	 	localField: 'host_id',
	 	foreignField: '_id',
	 	as: 'host_info'
	$unwind: {
		path: '$host_info',
		preserveNullAndEmptyArrays: true,
返回数据：
	"_id": ObjectId("5ef0737e381fbaacea727432"), 
    "host_id": ObjectId("5ef070ee381fbaacea727429"), 
    "name": "qysg10001", 
    "version": "37184", 
    "port": 80001, 
    "host_info": { 
    	"_id": ObjectId("5ef070ee381fbaacea727429"), 
    	"host": "hdss7-21", 
    	"private_ip": "172.16.7.21", 
    	"public_ip": "202.100.23.21", 
    	"region_id": "杭州", 
    	"status": true 
3）现在通过$project对数据进行过滤，我们只需要拿到host_info下的host、private_ip和status信息
db.gameapp.aggregate([
	$match: {name: 'qysg10001'} 
	$lookup: {
	 	from: 'hosts',
	 	localField: 'host_id',
	 	foreignField: '_id',
	 	as: 'host_info'
	$unwind: {
		path: '$host_info',
		preserveNullAndEmptyArrays: true,
	$project: {
		_id: 0,
		'host_info.host': 1,
		'host_info.private_ip': 1,
		'host_info.status': 1,		
返回结果：
	"host_info": 
		"host": "hdss7-21", 
		"private_ip": "172.16.7.21", 
		"status": true