sqlalchemy中的python继承

Question

所以我是这个 python 和 sqlalchemy 的新手。我需要一些关于继承的帮助，或者可能需要一个 mixin（而是继承）。

我有一些伪代码，但我还没有真正取得任何进展：

Base = declarative_base()

class ModelBase(Base):
  """Base model that only defines last_updated"""
  __tablename__ = 'doesnotexistandtheclassshouldnotbeinstantiated'

  #all tables inheriting from ModelBase will have this column
  last_updated = Column(DateTime)

  def __init__(self, last_updated):
    self.last_updated = last_updated

class User(ModelBase):
  """Defines the user but should also have the last_updated inherited from ModelBase"""
  __tablename__ = 'user'

  id = Column(Integer, primary_key=True)

  def __init__(self, ....):
    ModelBase.__init__(last_updated)

我希望从 ModelBase 继承的所有表也都具有 last_updated。我该怎么做？

更新代码：

class BaseUserMixin(object):
    """Base mixin for models using stamped data"""

    @declared_attr
    def last_updated(cls):
        return Column(DateTime)

    @declared_attr        
    def last_updated_by(cls):
        return Column(String)

    def __init__(self, last_updated, last_updated_by):
        self.last_updated = last_updated
        self.last_updated_by = last_updated_by

Base = declarative_base(cls=BaseUserMixin)


class User(Base):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    email = Column(String)
    fullname = Column(String)
    password = Column(String)
    enabled = Column(Boolean)

    def __init__(self, name, fullname, password, email, last_updated, last_updated_by):
        self.name = name
        self.fullname = fullname
        self.password = password
        self.email = email
        # goes wrong here
        super(User, self).__init__(last_updated, last_updated_by)

    def __repr__(self):
        return "<User('%', '%', '%', '%', '%', '%')>"\
               % (self.name,
                  self.fullname,
                  self.password,
                  self.email,
                  self.last_updated,
                  self.last_updated_by
                  )

错误是：

_declarative_constructor() takes exactly 1 argument (3 given)

可能是什么问题？我以为它可以工作，但是当重新运行调试器时它失败了。

Answer 1

解决办法是declared_attr；任何时候都会调用并添加到 DeclarativeMeta 的实例中：

编辑：declarative自动提供的__init__不能调用super()。如果你想要它，它必须是 last，唯一的方法就是使用常规的 mixin。

import datetime
from sqlalchemy import Column, DateTime, Integer, String
from sqlalchemy.ext.declarative import declared_attr, declarative_base

class BaseMixin(object):
    @declared_attr
    def last_updated(cls):
        return Column(DateTime)

    def __init__(self, last_updated, *args, **kwargs):
        super(BaseMixin, self).__init__(last_updated=datetime.datetime.now(), *args, **kwargs)
        print "BaseMixin.__init__"
        self.last_updated = last_updated

ModelBase = declarative_base()

注意，mixin必须先出现！

class User(BaseMixin, ModelBase):
    """Defines the user but should also have the last_updated inherited from ModelBase"""
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)
    username = Column(String)

    def __init__(self, *args, **kwargs):
        super(User, self).__init__(last_updated=datetime.datetime.now(), *args, **kwargs)
        print "User.__init__"

if __name__ == '__main__':
    from sqlalchemy import create_engine
    from sqlalchemy.orm import sessionmaker
    engine = create_engine('sqlite:///:memory:', echo=True)
    ModelBase.metadata.create_all(engine)
    user = User(username='alice')

    Session = sessionmaker(engine)
    session = Session()
    session.add(user)
    session.commit()

但是；您确定要首先使用__init__ 吗？从查询返回对象时不调用__init__；而您真正想要的是在修改列时立即更改为。这已经融入了Column()：

from sqlalchemy import func

class BaseMixin(object):
    @declared_attr
    def created_date(cls):
        return Column(DateTime, default=func.now())

    @declared_attr
    def modified_date(cls):
        return Column(DateTime, default=func.now(), onupdate=func.now())

回到使用 cls= 参数

ModelBase = declarative_base(cls=BaseMixin)


class User(ModelBase):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)
    username = Column(String)

if __name__ == '__main__':
    engine = create_engine('sqlite:///:memory:', echo=True)
    ModelBase.metadata.create_all(engine)
    user = User(username='alice')

    Session = sessionmaker(engine)
    session = Session()
    session.add(user)
    session.commit()

    session = Session()
    sameuser = session.query(User).one()
    sameuser.username = 'bob'
    session.commit()