SQLAlchemy ORM - 即时加载

即时加载减少了查询次数。SQLAlchemy 提供通过查询选项调用的即时加载函数，这些函数为查询提供了附加指令。这些选项决定如何通过 Query.options() 方法加载各种属性。

子查询加载

我们希望 Customer.invoices 能够即时加载。orm.subqueryload() 选项提供了第二个 SELECT 语句，该语句完全加载与刚刚加载的结果相关的集合。名称"子查询"导致 SELECT 语句直接通过重用查询构建，并作为子查询嵌入到针对相关表的 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))

上述程序的输出如下−

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

joinedload

另一个函数称为 orm.joinedload()。这将发出 LEFT OUTER JOIN。Lead 对象以及相关对象或集合将在一个步骤中加载。

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

这将发出以下表达式，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，但它返回了一个 Customer 实例。这是因为 Query 对返回的实体应用了基于对象标识的"唯一"策略。可以应用连接的预先加载而不会影响查询结果。

subqueryload() 更适合加载相关集合，而 joinload() 更适合多对一关系。