如何根据SqlAlchemy关系设置模型默认值？

Posted 2021-04-25

假设我有以下型号：

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默认为Treatment.treatment_date前一天。换句话说，我想做以下事情：

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

如何通过q​​azxswpoi等方法动态设置默认值时，如何根据关系设置默认值？

---

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

append

但是抛出TypeError是因为SqlAlchemy还没有设置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) 字段。

答案

似乎SQLAlchemy在模型添加并提交到会话之前不会双向配置关系。

self.customer

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

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>