<p>在SQL中,外键<em>必须映射到一个特定的表,因此需要将外键放在指向'car'或'suv'表中信息id'. 在</p>
<p>对于你所需要的,这可能有点过分了,但是有一种方法可以解决它(假设你确实希望每辆车都只有一个信息):</p>
<pre><code>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'
</code></pre>
<p>事件侦听器的复杂性使您能够在执行以下操作时自动管理类型:</p>
^{pr2}$
<p>或者</p>
<pre><code>info1 = car1.info
info1.suv = suv1
assert (car1.info is None)
assert (info1.type == 'suv')
</code></pre>
<p>如果你想留下信息类型, 信息车,和信息suv一致性,您可以省略所有事件侦听器函数。在</p>
<p>为CarInfo和SuvInfo单独设置对象和表也是一个非常合理的选择,并完全避免所有这些复杂性。在</p>