Flask-SQLAlchemy：CircularDependencyError其中多对一关系中的同一行可以与同一表中的一对多关系

Question

我使用Flask-SQLAlchemy 2.1和SQLAlchemy 1.0.13，并且我有两个表， Address和Customer有彼此多重关系，具体如下：

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' 
    ) 

还有两个事件侦听器，可以自动添加任何设置billing_address或shipping_address到addresses为Customer实例：

@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) 

试图设置Customer.billing_address和Customer.shipping_address导致CircularDependencyError因为我希望：

> 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>)) 

如果我注释掉的事件监听器，这不会导致CircularDependencyError，这也是我所期望的，因为Customer.address没有被访问。然而，这不是一种解决方案，因为在billing_address或shipping_address和addresses中存在相同的Address实例导致的循环依赖性，我希望允许addresses包含当前帐单和送货地址。

按照relevant SQLAlchemy docs这应该是可以解决的，加入的关系post_update=True参数一侧，并给予它的外键的名称：但

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 
    ) 

这仍然引发了CircularDependencyError，：

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)) 

我也试过将use_alter=True传递给customer_id外键，如某些相关的StackOverflow帖子中提到的：

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

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

Answer 1

设置上关系的双方post_update=True出现解决了问题：当添加一个billing_address和/或shipping_address

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，它会自动添加到addresses没有问题。添加新的billing_address或shipping_address的行为与我预期的一样，将旧地址留在addresses以及添加新地址。

但是，我不完全相信这个答案，因为SQLAlchemy文档明确提到post_update=True应该设置为关系的一方，而不是两个，所以我想知道我的解决方案是否会导致意外的行为。

编辑 - 下面是一个正确的解决方案：

出于某种原因，设置在addressespost_update=True不能设置在customer（反之亦然）的不是工作，而是把它放在billing_address和shipping_address的建议由@univerio。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 
    )