$lookup 是 Mongodb 3.2版本 新增的聚合框架中的一种查询方式； 主要用来实现多表链接查询； 相当关系型数据库中多表链接查询。
- 主要功能 是将每个输入待处理的文档，经过$lookup 阶段的处理，输出的新文档中会包含一个新生成的数组列（户名可根据需要命名新key的名字 ）。数组列存放的数据 是 来自 被Join 集合的适配文档，如果没有，集合为空（即 为[ ]）
| 字段 | 描述 |
|---|---|
| from | 同一个database中用于连接查询的collection的名称 |
| localField | 源集合中的match值，如果输入的集合中，某文档没有 localField这个Key（Field），在处理的过程中，会默认为此文档含有 localField：null的键值对。 |
| foreignField | 存在于from那个集合的外键 |
| as | 别名 |
| let | 定义参数，在pipeline中使用 |
| pipeline | 对join进来的数据做筛选 |
- 例子、插入orders数据
python
import pymongo
import random
import string
from pymongo.collation import Collation
random.seed(10)
letters = string.ascii_lowercase
upper = string.ascii_uppercase
class MongoDBServer():
def __init__(self,database,collation) -> None:
self.client = pymongo.MongoClient('mongodb://ellis:[email protected]:32000/')
self.database = self.client[database]
self.colleceion = self.database[collation]
def insert_many(self,documents):
self.colleceion.insert_many(documents)
server = MongoDBServer('lookup','orders')
server.insert_many([
{ "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 },
{ "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 },
{ "_id" : 3 }
] )插入inventory数据
python
import pymongo
import random
import string
from pymongo.collation import Collation
random.seed(10)
letters = string.ascii_lowercase
upper = string.ascii_uppercase
class MongoDBServer():
def __init__(self,database,collation) -> None:
self.client = pymongo.MongoClient('mongodb://ellis:[email protected]:32000/')
self.database = self.client[database]
self.colleceion = self.database[collation]
def insert_many(self,documents):
self.colleceion.insert_many(documents)
def lookup(self,destination,localField,foreignField,as_field):
return self.colleceion.aggregate([{"$lookup":{"from":destination,"localField":localField,"foreignField":foreignField,"as":as_field}}])
server = MongoDBServer('lookup','inventory')
server.insert_many([
{ "_id" : 1, "sku" : "almonds", "description": "product 1", "instock" : 120 },
{ "_id" : 2, "sku" : "bread", "description": "product 2", "instock" : 80 },
{ "_id" : 3, "sku" : "cashews", "description": "product 3", "instock" : 60 },
{ "_id" : 4, "sku" : "pecans", "description": "product 4", "instock" : 70 },
{ "_id" : 5, "sku": None, "description": "Incomplete" },
{ "_id" : 6 }
] )inventory 和orders通过item以及sku两个字段关联
- 使用lookup查询
python
import pymongo
import random
import string
from pymongo.collation import Collation
random.seed(10)
letters = string.ascii_lowercase
upper = string.ascii_uppercase
class MongoDBServer():
def __init__(self,database,collation) -> None:
self.client = pymongo.MongoClient('mongodb://ellis:[email protected]:32000/')
self.database = self.client[database]
self.colleceion = self.database[collation]
def insert_many(self,documents):
self.colleceion.insert_many(documents)
def lookup(self,destination,localField,foreignField,as_field):
return self.colleceion.aggregate([{"$lookup":{"from":destination,"localField":localField,"foreignField":foreignField,"as":as_field}}])
server = MongoDBServer('lookup','orders')
for item in server.lookup("inventory","item","sku","inventory_docs"):
print(item)- lookup 与array类型一起使用
python
import pymongo
import random
import string
from pymongo.collation import Collation
random.seed(10)
letters = string.ascii_lowercase
upper = string.ascii_uppercase
class MongoDBServer():
def __init__(self,database,collation) -> None:
self.client = pymongo.MongoClient('mongodb://ellis:[email protected]:32000/')
self.database = self.client[database]
self.colleceion = self.database[collation]
def insert_many(self,documents):
self.colleceion.insert_many(documents)
def lookup(self,destination,localField,foreignField,as_field):
return self.colleceion.aggregate([{"$lookup":{"from":destination,"localField":localField,"foreignField":foreignField,"as":as_field}}])
server = MongoDBServer('lookup','classes')
server.insert_many([
{ "_id": 1, "title": "Reading is ...", "enrollmentlist": [ "giraffe2", "pandabear", "artie" ], "days": ["M", "W", "F"] },
{ "_id": 2, "title": "But Writing ...", "enrollmentlist": [ "giraffe1", "artie" ], "days": ["T", "F"] }
] )
server = MongoDBServer('lookup','members')
server.insert_many([
{ "_id": 1, "name": "artie", "status": "A" },
{ "_id": 2, "name": "giraffe", "status": "D" },
{ "_id": 3, "name": "giraffe1", "status": "A" },
{ "_id": 4, "name": "panda", "status": "A" },
{ "_id": 5, "name": "pandabear", "status": "A" },
{ "_id": 6, "name": "giraffe2", "status": "D" }
] )
for item in server.lookup('members','enrollmentlist','name',"haha"):
print(item)- let以及pipeline的使用
python
import pymongo
import random
import string
from pymongo.collation import Collation
random.seed(10)
letters = string.ascii_lowercase
upper = string.ascii_uppercase
class MongoDBServer():
def __init__(self,database,collation) -> None:
self.client = pymongo.MongoClient('mongodb://ellis:[email protected]:32000/')
self.database = self.client[database]
self.colleceion = self.database[collation]
def insert_many(self,documents):
self.colleceion.insert_many(documents)
def lookup(self,destination,localField,foreignField,as_field):
return self.colleceion.aggregate([{"$lookup":{"from":destination,"localField":localField,"foreignField":foreignField,"as":as_field}}])
server = MongoDBServer('lookup','orders')
for item in server.colleceion.aggregate([
{
"$lookup":{
"from": "inventory",
"localField": "item",
"foreignField": "sku",
"let": {
"instock": 100
},
"pipeline": [
{ "$match":
{ "$expr":
{ "$gt": [ "$instock", "$$instock"] }
}
}
],
"as": "haha"
}
},
# {"$project":{"item":1,"price":1,"number":{"$size":"$haha"}}},
# {"$match":{"number":{"$gte":1}}}
{
"$match":{"$expr":{"$gte":[{"$size":"$haha"}, 1]}}
}
]):
print(item)https://www.mongodb.com/docs/manual/reference/operator/aggregation/lookup/