约束引用多态模型的列

Question

我有三个模型跟随单表继承（Car，BlueCar，RedCar）和一个与RedCar有关系的模型RedCarUser。我希望有一个限制，防止使用BlueCar id创建RedCarUser，更简洁：

redcar = RedCar(model="toyota")
bluecar = BlueCar(model="mazda")

# I want this to work
user = RedCarUser(name="Bob", car_id=redcar.id)

# But this to fail
user = RedCarUser(name="Bob", car_id=bluecar.id)

# Currently they are both allowed.

这就是模型的设置方式：

from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.dialects.postgres import ENUM
from sqlalchemy.orm import relationship

from base import Base
from enum import Enum


class Colors(Enum):
    RED = 'Red'
    BLUE = 'Blue'


class Car(Base):
    colors_enum = ENUM(
        Colors.BLUE.value,
        Colors.RED.value,
        name='car_colors'
    )

    id = Column(Integer, primary_key=True)
    model = Column(String(255), nullable=False)
    color = Column(colors_enum, nullable=False, server_default=Colors.BLUE.value)

    __tablename__ = "cars"
    __mapper_args__ = {
        'polymorphic_on': color,
    }

    def __init__(self, model, color=Colors.BLUE.value):
        self.model = model
        self.color = color


class BlueCar(Car):
    __mapper_args__ = {
        'polymorphic_identity': Colors.BLUE.value
    }

    def __init__(self, model, color=Colors.RED.value):
        super(RedCar, self).__init__(model, color)


class RedCar(Car):
    __mapper_args__ = {
        'polymorphic_identity': Colors.RED.value
    }

    users = relationship('RedCarUser', back_populates='car')

    def __init__(self, model, color=Colors.RED.value):
        super(RedCar, self).__init__(model, color)


class RedCarUser(Base):
    __tablename__ = "red_car_users"

    id = Column(Integer, primary_key=True)
    name = Column(String(255), nullable=False)
    car_id = Column(Integer, ForeignKey(RedCar.id), nullable=False)

    car = relationship('RedCar', foreign_keys=[car_id])

    def __init__(self, name, car_id):
        self.name = name
        self.car_id = car_id