SQLAlchemy ORM - 使用连接

现在我们有两个表，我们将了解如何同时对这两个表创建查询。要构建 Customer 和 Invoice 之间的简单隐式连接，我们可以使用 Query.filter() 将其相关列关联起来。下面，我们使用此方法一次加载 Customer 和 Invoice 实体：

from sqlalchemy.orm import sessionmaker
Session = sessionmaker(bind = engine)
session = Session()

for c, i in session.query(Customer, Invoice).filter(Customer.id == Invoice.custid).all():
   print ("ID: {} Name: {} Invoice No: {} Amount: {}".format(c.id,c.name, i.invno, i.amount))

SQLAlchemy 发出的 SQL 表达式如下：

SELECT customers.id 
AS customers_id, customers.name 
AS customers_name, customers.address 
AS customers_address, customers.email 
AS customers_email, invoices.id 
AS invoices_id, invoices.custid 
AS invoices_custid, invoices.invno 
AS invoices_invno, invoices.amount 
AS invoices_amount
FROM customers, invoices
WHERE customers.id = invoices.custid

以上代码行的结果如下：

ID: 2 Name: Gopal Krishna Invoice No: 10 Amount: 15000
ID: 2 Name: Gopal Krishna Invoice No: 14 Amount: 3850
ID: 3 Name: Govind Pant Invoice No: 3 Amount: 10000
ID: 3 Name: Govind Pant Invoice No: 4 Amount: 5000
ID: 4 Name: Govind Kala Invoice No: 7 Amount: 12000
ID: 4 Name: Govind Kala Invoice No: 8 Amount: 8500
ID: 5 Name: Abdul Rahman Invoice No: 9 Amount: 15000
ID: 5 Name: Abdul Rahman Invoice No: 11 Amount: 6000

可以使用 Query.join() 方法轻松实现实际的 SQL JOIN 语法，如下所示：

session.query(Customer).join(Invoice).filter(Invoice.amount == 8500).all()

连接的 SQL 表达式将显示在控制台上：

SELECT customers.id 
AS customers_id, customers.name 
AS customers_name, customers.address 
AS customers_address, customers.email 
AS customers_email
FROM customers JOIN invoices ON customers.id = invoices.custid
WHERE invoices.amount = ?

我们可以使用 for 循环遍历结果：

result = session.query(Customer).join(Invoice).filter(Invoice.amount == 8500)
for row in result:
   for inv in row.invoices:
      print (row.id, row.name, inv.invno, inv.amount)

以 8500 作为绑定参数，将显示以下输出：

4 Govind Kala 8 8500  

Query.join() 知道如何在这些表之间连接，因为它们之间只有一个外键。如果没有外键或多个外键，当使用以下其中一种形式时，Query.join() 效果更好：

类似地，outerjoin() 函数可用于实现左外连接。

query.outerjoin(Customer.invoices)

subquery() 方法生成一个 SQL 表达式，表示嵌入别名中的 SELECT 语句。

from sqlalchemy.sql import func

stmt = session.query(
   Invoice.custid, func.count('*').label('invoice_count')
).group_by(Invoice.custid).subquery()

stmt 对象将包含如下 SQL 语句：

SELECT invoices.custid, count(:count_1) AS invoice_count FROM invoices GROUP BY invoices.custid

获得语句后，它的行为类似于 Table 构造。语句上的列可以通过名为 c 的属性访问，如下面的代码所示：

for u, count in session.query(Customer, stmt.c.invoice_count).outerjoin(stmt, Customer.id == stmt.c.custid).order_by(Customer.id):
   print(u.name, count)

上面的 for 循环按名称显示发票计数，如下所示：

Arjun Pandit None
Gopal Krishna 2
Govind Pant 2
Govind Kala 2
Abdul Rahman 2