gistfile1.py

class AppendOnlyModelMeta(DeclarativeMeta):
    @staticmethod
    def _get_append_only_attributes(cls):
        '''A helper method to get a list of append-only attributes
        declared on the class.'''
        [...]

    def __init__(cls, classname, bases, dict_):
        if hasattr(cls, '__history_class__'):
            cls._history_fields = cls._get_append_only_attributes(cls)
            cls._history = relationship(
                cls.__history_class__,
                order_by=cls.__history_class__.id.desc())
        super(AppendOnlyModelMeta, cls).__init__(classname, bases, dict_)


class AppendOnlyModel(Base):
    '''The base class for all models that support append only storage.

    Subclasses need to implement '__history_class__' attribute
    pointing to a History table instance (inherited from HistoryBase)
    '''
    __abstract__ = True
    __metaclass__ = AppendOnlyModelMeta