基于记录数据的SqlAlchemy模型关联

Question

有一个info表，它与car表或suv表有关系。

在info.type字段中指定。

那么，如何根据记录的类型数据创建动态关联呢？

class Info(Base):
    item_id = Column(ForeignKey('cars-or-suvs-table.id'))
    type = Column(String())

class Car(Base):
    - data - 

    info = relationship('Info', backref="car")

class Suv(Base):
    - data - 

    info = relationship('Info', backref="suv")

编辑：--我已经有了填充了数据的表，所以我不能更改数据库模式。

Answer 1

由于您正在寻找不需要将外键移动到其他表的解决方案，因此可以尝试以下方法：

import sqlalchemy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, String, Integer
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy import sql
Base = declarative_base()
engine = sqlalchemy.create_engine('sqlite:///:memory:')
Session = sessionmaker(bind=engine)
session = Session()

class Info(Base):
    __tablename__ = 'info'
    id = Column(Integer(), primary_key=True)
    type = Column(String())
    item_id = Column(Integer())

    @property
    def item(self):
        if self.type == 'car':
            return self._car
        elif self.type == 'suv':
            return self._suv
        return None

    @item.setter
    def item(self, value):
        if value is not None:
            self.item_id = value.id
            self.type = value.__tablename__
        else:
            self.item_id = None

class Car(Base):
    __tablename__ = 'car'
    id = Column(Integer(), primary_key=True)
    info = relationship(Info, primaryjoin=sql.and_(id == Info.item_id, Info.type == 'car'), foreign_keys=Info.item_id, uselist=False, backref='_car')

class Suv(Base):
    __tablename__ = 'suv'
    id = Column(Integer(), primary_key=True)
    info = relationship(Info, primaryjoin=sql.and_(id == Info.item_id, Info.type == 'suv'), foreign_keys=Info.item_id, uselist=False, backref='_suv')

我将Info.car改名为Info._car，因为即使.type是'suv‘，._car也不可避免地是一个假的汽车对象。

为了保持简单，我把事件侦听器的内容省去了，但是您肯定可以从我的另一个答案中调整您需要的部分，以避免事情陷入不一致的状态。

Answer 2

在SQL中，外键必须映射到一个特定的表中，因此您需要将外键放在指向'info.id‘的“car”或“suv”表中。

对于您所需的内容，这可能有些过分，但这里有一种解决方法(假设您实际上希望每一辆汽车都只有一个信息)：

import sqlalchemy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, String, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy import event
Base = declarative_base()

class Info(Base):
    __tablename__ = 'info'
    id = Column(Integer(), primary_key=True)
    type = Column(String())
    # NOTE: can't use backref='info' because we need the attributes defined
    #   directly on both classes so we can attach event listeners
    car = relationship('Car', back_populates='info', uselist=False)
    suv = relationship('Suv', back_populates='info', uselist=False)

    @property
    def item(self):
        # could check self.type here if you wanted
        return self.car or self.suv

    @item.setter
    def item(self, value):
        if isinstance(value, Car):
            self.car = value
        elif isinstance(value, Suv):
            self.suv = value
        elif value is None:
            self.car = None
            self.suv = None
        else:
            raise ValueError("item must be Car or Suv")

@event.listens_for(Info.car, 'set')
def _car_set_event(target, value, oldvalue, initiator):
    if value is not None:
        target.type = 'car'
        if target.suv:
            target.suv = None
    elif target.type == 'car':
        target.type = None

@event.listens_for(Info.suv, 'set')
def _suv_set_event(target, value, oldvalue, initiator):
    if value is not None:
        target.type = 'suv'
        if target.car:
            target.car = None
    elif target.type == 'suv':
        target.type = None

class Car(Base):
    __tablename__ = 'car'
    id = Column(Integer(), primary_key=True)
    info_id = Column(Integer(), ForeignKey('info.id'))
    info = relationship(Info, back_populates='car')

@event.listens_for(Car.info, 'set')
def _car_info_set_event(target, value, oldvalue, initiator):
    if value is not None:
        value.type = 'car'

class Suv(Base):
    __tablename__ = 'suv'
    id = Column(Integer(), primary_key=True)
    info_id = Column(Integer(), ForeignKey('info.id'))
    info = relationship(Info, back_populates='suv')

@event.listens_for(Suv.info, 'set')
def _suv_info_set_event(target, value, oldvalue, initiator):
    if value is not None:
        value.type = 'suv'

事件侦听器的复杂性在于，当您执行以下操作时，将自动管理该类型：

car1.info = Info()
assert (car1.info.type == 'car')

或

info1 = car1.info
info1.suv = suv1
assert (car1.info is None)
assert (info1.type == 'suv')

如果希望保持Info.type、Info.car和Info.suv的一致性，可以省略所有事件侦听器函数。

为CarInfo和SuvInfo设置单独的对象和表，并完全避免所有这些复杂性，也是一个非常合理的选择。