SQLAlchemy:声明性Mixin类中的getter/setter

Question

我正在尝试为我打算在我的数据库模式中使用的mixin类定义简单的getter/setter方法:

from sqlalchemy import Column, Integer, create_engine
from sqlalchemy.orm import synonym, scoped_session, sessionmaker
from sqlalchemy.ext.declarative import declarative_base, declared_attr

engine = create_engine('sqlite:///')
Base = declarative_base(bind=engine)
Session = scoped_session(sessionmaker(bind=engine))


class Mixin(object):
    _attr = Column('attr', Integer)

    @property
    def attr(self):
        return self._attr

    @attr.setter
    def attr(self, value):
        self._attr = value

    attr = synonym('_attr', descriptor=attr)


class DummyClass(Base, Mixin):
    __tablename__ = 'dummy'

    id = Column(Integer, primary_key=True)

Base.metadata.create_all()

if __name__ == '__main__':
    session = Session()
    testobj = DummyClass()
    session.add(testobj)
    testobj.attr = 42
    assert testobj.attr == 42

当我尝试运行此示例时,我收到以下错误:

sqlalchemy.exc.InvalidRequestError:Mapper属性(即deferred,column_property(),relationship()等)必须在声明性mixin类中声明为@declared_attr callables.

我的代码几乎是SQLAlchemy Declarative Tutorial的1:1副本,唯一的区别是在Mixin类中声明了属性/同义词.将"@declared_attr"装饰器附加或添加到现有装饰器不会改变任何内容.

我该如何解决这种情况？

Answer 1

也许创建attr()一个@declared_attr返回同义词的-decorated类方法？

class Mixin(object):

    _attr = Column('attr', Integer)

    def get_attr(self):
        return self._attr

    def set_attr(self, value):
         self._attr = value

    @declared_attr
    def attr(cls):
        return synonym('_attr', descriptor=property(cls.get_attr, cls.set_attr))