如何為從sqlalchemy聲明基礎擴展的類定義元類

Question

我使用：Python 2.6和sqlalchemy 0.6.1

這就是我想要做的：

from sqlalchemy.types import (
    Integer,
    String,
    Boolean
)
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class SampleMeta(type):
    def __new__(cls, name, bases, attrs):
        attrs.update({   'id': Column('Id', Integer, primary_key=True),
                    'name': Column('Name', String),
                    'description': Column('Description', String),
                    'is_active': Column('IsActive', Boolean)
                })
        return super(SampleMeta, cls).__new__(cls, name, bases, attrs)

class Sample(Base):
    __tablename__ = 'Sample'
    __table_args__ = {'useexisting': True}
    __metaclass__ = SampleMeta

    def __init__(self, id, name, description, is_active):
        self.id = id
        self.name = name
        self.description = description
        self.is_active = is_active

    def __repr__(self):
        return "<(%d, '%s', '%s', %r)>" % (self.id, self.name, self.description, self.isactive)

我得到的錯誤是這樣的：

TypeError: Error when calling the metaclass bases
    metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

現在，如果我通過使用上面做同樣的事情

class Sample(object)

代替

class Sample(Base)

它工作得很好。

我需要動態更新類的屬性。 所以，我將使用動態屬性和列名稱。 我需要上面的代碼才能工作才能到達那里。

請幫忙

Answer 1

要使用元類，在這種情況下，它必須從Declaratives DeclaredMeta派生。 但是這個用例不需要元類。 使用mixins ：

from sqlalchemy.types import (
    Integer,
    String,
    Boolean
)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column

Base = declarative_base()

class SampleMixin(object):
    id = Column("Id", Integer, primary_key=True)
    name = Column("Name", String)
    description = Column("Description", String)
    is_active = Column("IsActive", Boolean)

class Sample(SampleMixin, Base):
    __tablename__ = 'Sample'
    __table_args__ = {'useexisting': True}

    def __init__(self, id, name, description, is_active):
        self.id = id
        self.name = name
        self.description = description
        self.is_active = is_active

    def __repr__(self):
        return "<(%d, '%s', '%s', %r)>" % (self.id, self.name,
                                self.description, self.isactive)

from sqlalchemy import select
print select([Sample.id, Sample.name])

Answer 2

如果由zzzeek提出多重繼承無法解決您的問題，請嘗試從DeclarativeMeta繼承您的MetaClass：

from sqlalchemy.ext.declarative import declarative_base, DeclarativeMeta

Base = declarative_base()

class SampleMeta(DeclarativeMeta):
    #...

class Sample(Base):
    __metaclass__ = SampleMeta
    #...