如何基于SqlAlchemy关系设置模型默认值？

Question

假设我有以下模型：

class Customer(Model):
    __tablename__ = 'customer'
    id = Column(Integer())
    treatments = relationship('Treatment', back_populates='customer')
    shipments = relationship('Shipment', back_populates='customer')

class Treatment(Model):
    __tablename__ = 'treatment'
    customer_id = Column(Integer(), ForeignKey('customer.id'))
    customer = relationship('Customer', back_populates='treatments')
    treatment_date = Column(DateTime(), nullable=False)

class Shipment(Model):
    __tablename__ = 'shipment'
    customer_id = Column(Integer(), ForeignKey('customer.id'))
    customer = relationship('Customer', back_populates='shipments')
    ship_date = Column(DateTime(), nullable=False)

我希望能够将Shipment.ship_date默认设置为Treatment.treatment_date的前一天。 换句话说，我要执行以下操作：

customer = Customer()
treatment = Treatment(treatment_date="11/02/2017")
customer.treatments.append(treatment)
shipment = Shipment()
customer.shipments.append(shipment)
shipment.ship_date
# 11/01/2017

当通过诸如append类的方法动态设置默认值时，如何根据默认值设置默认值？

---

为了澄清起见，这是关于SqlAlchemy以及何时建立关系的问题。 例如，我尝试了以下方法：

class Shipment(Model):
    # ...same set up as above
    def __init__(self, **kwargs):
        super().__init__(**kwargs)
        self.ship_date = self.customer.treatments[0].treatment_date - timedelta(1)

但这会引发TypeError，因为SqlAlchemy尚未设置self.customer字段。

Answer 1

似乎SQLAlchemy不会在两种情况下都配置关系，直到将模型添加并提交到会话之后。

customer.shipments.append(new_shipment)
customer.shipments # includes the new shipment
new_shipment.customer # None
session.add(customer)
session.commit()
new_shipment.customer # <Customer object>

虽然这很烦人，而且我不确定为什么在创建一侧时SQLAlchemy不会填充双向关系，但是可以通过手动设置两侧的关系来解决。 例如：

new_shipment = Shipment(customer=customer)
new_shipment.ship_date # 11/01/2017
customer.shipments.append(new_shipment)