如何基于SqlAlchemy关系设置模型默认值？

Question

Let's say I have the following models:

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)

I would like to be able to default the Shipment.ship_date to be the day before the Treatment.treatment_date. In other words, I want to do the following:

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

How do I set defaults based on relationships when they're set dynamically by methods like append ?

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For clarification, this is a question about SqlAlchemy and when relationships are set up. For example, I've tried the following:

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)

But that throws a TypeError because SqlAlchemy hasn't set up the self.customer field yet.

Answer 1

It appears as though SQLAlchemy does not configure relationships both ways until after the model has been added and committed to the session.

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>

While this is annoying and I'm not sure why SQLAlchemy doesn't populate the two-way relationship when one side is created, it can be resolved by manually setting the relationship on both sides. For example:

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