的SQLAlchemy：吸氣劑聲明混合類/ setter方法

Question

我試圖定義一個混合類簡單的getter/setter方法，我打算在我的數據庫使用的模式：

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 

當我嘗試運行這個例子，我得到以下錯誤：

sqlalchemy.exc.InvalidRequestError: Mapper properties (i.e. deferred,column_property(), relationship(), etc.) must be declared as @declared_attr callables on declarative mixin classes.`

我的代碼幾乎是1：從SQLAlchemy Declarative Tutorial 1份，與物業/同義詞是在混合類中聲明的唯一區別。將「@declared_attr」裝飾器追加或預先添加到現有的裝飾器不會改變任何內容。

我該如何解決這種情況？

Answer 1

也許創建attr()作爲@declared_attr-裝飾的類方法，返回的同義詞？

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))