mongodb多表查询(附带pymongo实例)

mongodb有$lookup可以做多表查询

举个例子

数据如下

db.orders.insert([
   { "_id" : , "item" : "almonds", "price" : , "quantity" :  },
   { "_id" : , "item" : "pecans", "price" : , "quantity" :  },
   { "_id" :   }
])
db.inventory.insert([
   { "_id" : , "sku" : "almonds", description: "product 1", "instock" :  },
   { "_id" : , "sku" : "bread", description: "product 2", "instock" :  },
   { "_id" : , "sku" : "cashews", description: "product 3", "instock" :  },
   { "_id" : , "sku" : "pecans", description: "product 4", "instock" :  },
   { "_id" : , "sku": null, description: "Incomplete" },
   { "_id" :  }
])

聚合操作如下

db.orders.aggregate([
 {
 $lookup:
 {
  from: "inventory",
  localField: "item",
  foreignField: "sku",
  as: "inventory_docs"
 }
 },
])

上面的代码意思是，从order表中取item字段作为inventory表中sku的查询条件，然后把数据保存到inventory_docs字段,

等价于mysql中的

SELECT *, inventory_docs
FROM orders
WHERE inventory_docs IN (SELECT *
FROM inventory
WHERE sku= orders.item);

下面用pymongo实现

from pymongo import MongoClient
 
def test():
    client = MongoClient()
    db = client[db_name]
    db['orders'].aggregate([{'$lookup':{'from': "inventory", "localField": "item", "foreignField": "sku", "as": "inventory_docs"}}])

这样就实现了上述的效果了

上述操作返回以下结果

{
   "_id" : ,
   "item" : "almonds",
   "price" : ,
   "quantity" : ,
   "inventory_docs" : [
      { "_id" : , "sku" : "almonds", "description" : "product 1", "instock" :  }
   ]
}
{
   "_id" : ,
   "item" : "pecans",
   "price" : ,
   "quantity" : ,
   "inventory_docs" : [
      { "_id" : , "sku" : "pecans", "description" : "product 4", "instock" :  }
   ]
}
{
   "_id" : ,
   "inventory_docs" : [
      { "_id" : , "sku" : null, "description" : "Incomplete" },
      { "_id" :  }
   ]
}

看起来蛮方便的，但是其实很麻烦，如果我们只想显示inventory表中的某些字段，这样不符合我们的要求

这时候就需要用到$project和$arrayElemAt

如果我们只想显示inventory中的instock字段的话应该这样做

db.orders.aggregate([
 {
 $lookup:
 {
  from: "inventory",
  localField: "item",
  foreignField: "sku",
  as: "inventory_docs"
 }
 },
 {
 $project:
 {
    'instock': {'$arrayElemAt':['$inventory_docs.instock', ]}
  },
 }，
])

返回的结果是

{'_id': 1.0, 'instock': 120.0}
{'_id': 2.0, 'instock': 70.0}
{'_id': 3.0}

之前orders的数据好像没了，不用着急，在project里面添加想要返回的数据项即可

如我想要返回orders的price字段，在$project中添加{'price':1}即可，其他同

lookup更多详细用法

project可以指定返回的内容

 $project:
 {
    'sku':, 'item':,
 }

指定返回sku和item

也可以重命名

$project:
{
    'test': '$sku',
    'item': ,
}

将sku字段重命名为test返回

https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/index.html