MongoDB 聚合管道（Aggregation Pipeline）

1. 聚合管道

使用聚合管道可对集合中的文档进行变换和组合。
应用场景：表关联查询、数据的统计。

MongoDB 中使用 db.COLLECTION_NAME.aggregate([{< stage >},…]) 方法 来构建和使用聚合管道。
先看下官网给的实例，感受一下聚合管道的用法。

2. 管道操作符与表达式

管道表达式：

管道操作符作为“键”，所对应的“值”叫做管道表达式。

{ $match:{status:"A"}}，$match 称为管道操作符，而 status:"A" 称为管道表达式， 是管道操作符的操作数(Operand)。 
每个管道表达式是一个文档结构，它是由字段名、字段值、和一些表达式操作符组成的。

3. 数据模拟

db.order.insert({"order_id":"1","uid":10,"trade_no":"111","all_price":100,"all_num":2}) 
db.order.insert({"order_id":"2","uid":7,"trade_no":"222","all_price":90,"all_num":2}) 
db.order.insert({"order_id":"3","uid":9,"trade_no":"333","all_price":20,"all_num":6}) 

db.order_item.insert({"order_id":"1","title":"商品鼠标 1","price":50,num:1})
db.order_item.insert({"order_id":"1","title":"商品键盘 2","price":50,num:1})
db.order_item.insert({"order_id":"1","title":"商品键盘 3","price":0,num:1}) 

db.order_item.insert({"order_id":"2","title":"牛奶","price":50,num:1}) 
db.order_item.insert({"order_id":"2","title":"酸奶","price":40,num:1}) 
db.order_item.insert({"order_id":"3","title":"矿泉水","price":2,num:5}) 
db.order_item.insert({"order_id":"3","title":"毛巾","price":10,num:1})

$project
作用：修改文档的结构，可以用来重命名、增加或删除文档中的字段；

要求查找 order 只返回文档中 trade_no 和 all_price 字段

db.order.aggregate([ { $project:{ trade_no:1, all_price:1 } } ])

$match
作用：用于过滤文档。用法类似于 find() 方法中的参数。

db.order.aggregate([ { $project:{ trade_no:1, all_price:1 } }, { $match:{"all_price":{$gte:90}} } ])

$group
作用：将集合中的文档进行分组，可用于统计结果

统计每个订单的订单数量，按照订单号分组

db.order_item.aggregate( [ { $group: {_id: "$order_id", total: {$sum: "$num"}} } ])

$sort
作用：将集合中的文档进行排序

db.order.aggregate([ 
	{ $project:{ trade_no:1, all_price:1 } }, 
	{ $match:{"all_price":{$gte:90}} },
	{ $sort:{"all_price":-1} } 
])

$limit
作用：限制结果的数量

db.order.aggregate([ 
	{ $project:{ trade_no:1, all_price:1 } }, 
	{ $match:{"all_price":{$gte:90}} }, 
	{ $sort:{"all_price":-1} }, 
	{ $limit:1 } 
])

$skip
作用：跳过文档的数量

db.order.aggregate([ 
	{ $project:{ trade_no:1, all_price:1 } }, 
	{ $match:{"all_price":{$gte:90}} }, 
	{ $sort:{"all_price":-1} },
	{ $skip:1 } 
])

$lookup
作用：表关联

eg1：

db.order.aggregate([ 
 { 
 	$lookup: { 
 	  from: "order_item", 
 	  localField: "order_id",  # 主表
 	  foreignField: "order_id", 
 	  as: "items" 
 	} 
 } 
])

结果：

{ 
	"_id": ObjectId("5b743d8c2c327f8d1b360540"), 
	"order_id": "1", 
	"uid": 10, 
	"trade_no": "111", 
	"all_price": 100, 
	"all_num": 2, 
	"items": [
		{ "_id": ObjectId("5b743d9c2c327f8d1b360543"), "order_id": "1", "title": "商品鼠标 1", "price": 50, "num": 1 }, 
		{ "_id": ObjectId("5b743da12c327f8d1b360544"), "order_id": "1", "title": "商品键盘 2", "price": 50, "num": 1 }, 
		{ "_id": ObjectId("5b74f457089f78dc8f0a4f3b"), "order_id": "1", "title": "商品键盘 3", "price": 0, "num": 1 }
	] 
} ,{ 
	"_id": ObjectId("5b743d902c327f8d1b360541"), 
	"order_id": "2", 
	"uid": 7, 
	"trade_no": "222", 
	"all_price": 90, 
	"all_num": 2, 
	"items": [
		{ "_id": ObjectId("5b743da52c327f8d1b360545"), "order_id": "2", "title": "牛奶", "price": 50, "num": 1 }, 
		{ "_id": ObjectId("5b743da92c327f8d1b360546"), "order_id": "2","title": "酸奶", "price": 40, "num": 1 }
	]
}, { 
	"_id": ObjectId("5b743d962c327f8d1b360542"),  
	"order_id": "3", 
	"uid": 9, 
	"trade_no": "333", 
	"all_price": 20, 
	"all_num": 6, 
	"items": [
		{ "_id": ObjectId("5b743dad2c327f8d1b360547"), "order_id": "3", "title": "矿泉水", "price": 2, "num": 5 }, 
		{ "_id": ObjectId("5b743dff2c327f8d1b360548"), "order_id": "3", "title": "毛巾", "price": 10, "num": 1 }
	] 
}

eg2：

db.order.aggregate([ 
  { 
	$lookup: { 
	  from: "order_item", 
	  localField: "order_id", 
	  foreignField: "order_id", 
	  as: "items" 
	} 
  }, 
  { 
  	$project:{ trade_no:1, all_price:1,items:1 } 
  }, 
  { 
  	$match:{"all_price":{$gte:90}} 
  }, 
  { 
  	$sort:{"all_price":-1}
  }, 
])

结果：

{ 
	"_id": ObjectId("5b743d8c2c327f8d1b360540"), 
	"trade_no": "111", 
	"all_price": 100, 
	"items": [
		{ "_id": ObjectId("5b743d9c2c327f8d1b360543"), "order_id": "1", "title": "商品鼠标 1", "price": 50, "num": 1 }, 
		{"_id": ObjectId("5b743da12c327f8d1b360544"), "order_id": "1", "title": "商品键盘 2", "price": 50, "num": 1 }, 
	    {"_id": ObjectId("5b74f457089f78dc8f0a4f3b"), "order_id": "1", "title": "商 品键盘 3", "price": 0, "num": 1 }
	] 
}, { 
	"_id": ObjectId("5b743d902c327f8d1b360541"), 
	"trade_no": "222", 
	"all_price": 90, 
	"items": [
		{ "_id": ObjectId("5b743da52c327f8d1b360545"), "order_id": "2", "title": "牛奶", "price": 50, "num": 1 }, 
		{"_id": ObjectId("5b743da92c327f8d1b360546"), "order_id": "2", "title": "酸奶", "price": 40, "num": 1}
	] 
}