SQLAlchemy 中的多个连接路径

Question

我尝试测试 SQLAlchemy 文档中有关处理多个连接路径的代码示例。但是，在我创建了一个客户对象之后，两个关系属性都为无。我想知道如何正确处理多个连接路径？我是否也需要在 Address 类中创建关系？我什么时候需要使用 back_populates？

Handling Multiple Join Paths

from sqlalchemy import create_engine
from sqlalchemy import Integer, ForeignKey, String, Column
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

Base = declarative_base()

class Customer(Base):
    __tablename__ = 'customer'
    id = Column(Integer, primary_key=True)
    name = Column(String)

    billing_address_id = Column(Integer, ForeignKey("address.id"))
    shipping_address_id = Column(Integer, ForeignKey("address.id"))

    billing_address = relationship("Address", foreign_keys=[billing_address_id])
    shipping_address = relationship("Address", foreign_keys=[shipping_address_id])

class Address(Base):
    __tablename__ = 'address'
    id = Column(Integer, primary_key=True)
    street = Column(String)
    city = Column(String)
    state = Column(String)
    zip = Column(String)

engine = create_engine('sqlite:///Testing.db')

Base.metadata.create_all(engine)

a1 = Address(street="a street", city="a city", state="A", zip="12345")
a2 = Address(street="b street", city="b city", state="B", zip="1233")
c1 = Customer(name="Jack")
print(c1.billing_address)

Answer 1

Address 中的关系不是必需的，如果您不需要它们。 back_populates= 用于将 2 个关系显式链接在一起，或者正如文档所说，"establish “bidirectional” behavior between each other"。

您的客户和地址未链接的原因是您从不链接它们。在施工期间将地址传递给客户或在之后设置它们：

c1 = Customer(name="Jack", billing_address=a1)
c1.shipping_address = a2

现在，当您将c1 添加到会话并提交时，SQLAlchemy 将处理以正确顺序插入a1、a2 和c1，以便它可以填写c1 的外键属性.发生这种情况是因为默认情况下 relationship() 启用了 save-update 级联，这也将关联对象放置到会话中。