SQLAlchemy ORM-渴望加载

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📜 SQLAlchemy ORM-渴望加载

📅 最后修改于: 2020-11-27 07:53:30 🧑 作者: Mango

渴望的负载减少了查询数量。 SQLAlchemy提供了通过查询选项调用的紧急加载函数，这些函数为Query提供了更多说明。这些选项确定如何通过Query.options()方法加载各种属性。

子查询加载

我们希望Customer.invoices能够尽快加载。 orm.subqueryload()选项提供第二条SELECT语句，该语句完全加载与刚加载的结果关联的集合。名称“ subquery”使SELECT语句可以通过重新使用的Query直接构造，并作为子查询嵌入到相关表的SELECT中。

from sqlalchemy.orm import subqueryload
c1 = session.query(Customer).options(subqueryload(Customer.invoices)).filter_by(name = 'Govind Pant').one()

这导致以下两个SQL表达式-

SELECT customers.id 
AS customers_id, customers.name 
AS customers_name, customers.address 
AS customers_address, customers.email 
AS customers_email
FROM customers
WHERE customers.name = ?
('Govind Pant',)

SELECT invoices.id 
AS invoices_id, invoices.custid 
AS invoices_custid, invoices.invno 
AS invoices_invno, invoices.amount 
AS invoices_amount, anon_1.customers_id 
AS anon_1_customers_id
FROM (
   SELECT customers.id 
   AS customers_id
   FROM customers
   WHERE customers.name = ?) 
   
AS anon_1 
JOIN invoices 
ON anon_1.customers_id = invoices.custid 
ORDER BY anon_1.customers_id, invoices.id 2018-06-25 18:24:47,479 
INFO sqlalchemy.engine.base.Engine ('Govind Pant',)

要访问两个表中的数据，我们可以使用以下程序-

print (c1.name, c1.address, c1.email)

for x in c1.invoices:
   print ("Invoice no : {}, Amount : {}".format(x.invno, x.amount))

上面程序的输出如下-

Govind Pant Gulmandi Aurangabad gpant@gmail.com
Invoice no : 3, Amount : 10000
Invoice no : 4, Amount : 5000

联合负载

另一个函数称为orm.joinedload()。这会发出一个LEFT OUTER JOIN。引导对象以及相关对象或集合将一步加载。

from sqlalchemy.orm import joinedload
c1 = session.query(Customer).options(joinedload(Customer.invoices)).filter_by(name='Govind Pant').one()

这发出以下表达式，给出与上述相同的输出-

SELECT customers.id 
AS customers_id, customers.name 
AS customers_name, customers.address 
AS customers_address, customers.email 
AS customers_email, invoices_1.id 
AS invoices_1_id, invoices_1.custid 
AS invoices_1_custid, invoices_1.invno 
AS invoices_1_invno, invoices_1.amount 
AS invoices_1_amount

FROM customers 
LEFT OUTER JOIN invoices 
AS invoices_1 
ON customers.id = invoices_1.custid

WHERE customers.name = ? ORDER BY invoices_1.id
('Govind Pant',)

OUTER JOIN产生了两行，但是它返回了一个Customer实例。这是因为Query基于对象标识将“唯一化”策略应用于返回的实体。可以在不影响查询结果的情况下应用联合急切加载。

subqueryload()更适合于加载相关集合，而joinedload()更适合于多对一关系。