Flask-SQLAlchemy：CircularDependencyError，其中多对一关系中的同一行可以与同一张表存在一对多关系

Question

我正在使用 Flask-SQLAlchemy 2.1 和 SQLAlchemy 1.0.13，我有两个表，Address 和 Customer，它们之间有多个关系，如下所示：

class Address(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    ...  # Other rows including first_name, last_name, etc.
    customer_id = db.Column(
        db.Integer,
        db.ForeignKey('customers.id')
    )
    customer = db.relationship(
        'Customer',
        foreign_keys=customer_id,
        back_populates='addresses'
    )

class Customer(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    billing_address_id = db.Column(db.Integer, db.ForeignKey('addresses.id'))
    billing_address = db.relationship(
        'Address',
        foreign_keys=billing_address_id
    )
    shipping_address_id = db.Column(
        db.Integer,
        db.ForeignKey('addresses.id')
    )
    shipping_address = db.relationship(
        'Address',
        foreign_keys=shipping_address_id
    )
    addresses = db.relationship(
        'Address',
        foreign_keys='Address.customer_id',
        back_populates='customer'
    )

对于Customer 实例，还有两个事件侦听器会自动将任何集合billing_address 或shipping_address 添加到addresses：

@event.listens_for(Customer.billing_address, 'set')
def add_billing_address_event(target, value, oldvalue, initiator):
    """If a billing address is added to a `Customer`, add it to addresses."""
    if value is not None and value not in target.addresses:
        target.addresses.append(value)


@event.listens_for(Customer.shipping_address, 'set')
def add_shipping_address_event(target, value, oldvalue, initiator):
    """If a shipping address is added to `Customer`, add to addresses."""
    if value is not None and value not in target.addresses:
        target.addresses.append(value)

尝试设置 Customer.billing_address 和 Customer.shipping_address 会导致 CircularDependencyError 如我所料：

> c = Customer()
> c.billing_address = Address(first_name='Bill')
> c.shipping_address = Address(first_name='Ship')
> db.session.add(c)
> db.session.flush()

CircularDependencyError: Circular dependency detected. (ProcessState(ManyToOneDP(Customer.shipping_address), <Customer at 0x7f53aa5c9fd0>, delete=False), ProcessState(ManyToOneDP(Address.customer), <Address at 0x7f53aa4e4128>, delete=False), ProcessState(ManyToOneDP(Address.customer), <Address at 0x7f53aa4e4080>, delete=False), SaveUpdateState(<Customer at 0x7f53aa5c9fd0>), ProcessState(ManyToOneDP(Customer.billing_address), <Customer at 0x7f53aa5c9fd0>, delete=False), ProcessState(OneToManyDP(Customer.addresses), <Customer at 0x7f53aa5c9fd0>, delete=False), SaveUpdateState(<Address at 0x7f53aa4e4080>), SaveUpdateState(<Address at 0x7f53aa4e4128>))

如果我注释掉事件监听器，这不会导致CircularDependencyError，这也是我所期望的，因为Customer.address 未被访问。然而，这不是一个解决方案，因为循环依赖是由于在billing_address 或shipping_address 和addresses 中存在相同的Address 实例，我想允许addresses 包括当前的计费和运输地址。

根据relevant SQLAlchemy docs，这应该可以通过在关系的一侧添加post_update=True 参数并为其外键命名来解决：

class Address(db.Model):
    ...
    customer_id = db.Column(
        db.Integer,
        db.ForeignKey('customers.id', name='fk_customer_id')
    )
    customer = db.relationship(
        'Customer',
        foreign_keys=customer_id,
        back_populates='addresses',
        post_update=True
    )

这仍然会引发CircularDependencyError，但是：

CircularDependencyError: Circular dependency detected. (ProcessState(OneToManyDP(Customer.addresses), <Customer at 0x7f620af3ff60>, delete=False), SaveUpdateState(<Address at 0x7f620ae5a080>), SaveUpdateState(<Address at 0x7f620ae5a128>), ProcessState(ManyToOneDP(Customer.billing_address), <Customer at 0x7f620af3ff60>, delete=False), SaveUpdateState(<Customer at 0x7f620af3ff60>), ProcessState(ManyToOneDP(Customer.shipping_address), <Customer at 0x7f620af3ff60>, delete=False))

我还尝试将 use_alter=True 传递给 customer_id 外键，正如一些相关 StackOverflow 帖子中提到的那样：

customer_id = db.Column(
    db.Integer,
    db.ForeignKey('customers.id', name='fk_customer_id', use_alter=True)
)

CircularDependencyError 仍然会发生。我找到了一个似乎可行的解决方案，我将在下面发布，但我不确定它是否是正确的解决方案。

Answer 1

在关系两边设置post_update=True似乎可以解决问题：

class Address(db.Model):
    ...
    customer_id = db.Column(db.Integer, db.ForeignKey('customers.id'))
    customer = db.relationship(
        'Customer',
        foreign_keys=customer_id,
        back_populates='addresses',
        post_update=True
    )

class Customer(db.Model):
    ...
    addresses = db.relationship(
        'Address',
        foreign_keys='Address.customer_id',
        back_populates='customer',
        post_update=True
    )

现在当添加 billing_address 和/或 shipping_address 时，它会自动添加到 addresses 而不会出现问题。添加新的billing_address 或shipping_address 的行为也符合我的预期，将旧地址保留在addresses 中并添加新地址。

但是，我对这个答案并不完全有信心，因为 SQLAlchemy 文档明确提到应该为关系的一侧设置post_update=True，而不是两者，所以我想知道我的解决方案是否会导致意外行为。

编辑 - 这是正确的解决方案：

由于某种原因，在 addresses 上设置 post_update=True 而不同时在 customer 上设置（反之亦然）不起作用，但按照 @univerio 的建议将其设置在 billing_address 和 shipping_address 上。谢谢！

class Customer(db.Model):
...
    billing_address = db.relationship(
        'Address',
        foreign_keys=billing_address_id,
        post_update=True
    )
    shipping_address = db.relationship(
        'Address',
        foreign_keys=shipping_address_id,
        post_update=True
    )