在SQLAlchemy中,我已经阅读了如何在声明声明时组合来自不同mixin的__table_args__.

Combining Table/Mapper Arguments from Multiple Mixins

我的问题是,该示例显示了如何在链的末尾(MRO中的最终类)完成此操作,但如果我有这些Mixins并希望它出现在MyClientMixin或Base中,那将如何实现这一点类以避免为其他类型的mixin复制此代码？

class LaneCarrierCommitmentSummaryMixin(object):
    """ Lane Carrier Commitment Summary.

    A base class for LCCS to mixin with a client specific class.
    """

    __tablename__ = 'lane_carrier_commitment_summary'
    __table_args__ = ((UniqueConstraint(['hashmap_key', 'bow'],
                                        name='uq_lane_carrier_commitment_summary_hashmap_key_bow')),)

class MyClientMixin(object):
    """ MyClient Mixin class for providing the ETL schema. """

    __table_args__ = {'schema': 'myclient_etl'}

class MyClientLaneCarrierCommitmentSummary(LaneCarrierCommitmentSummaryMixin, DateTrackedMixin, MyClientMixin, Base):
    pass

我对这个概念有点挣扎.

解决方法:

此基类将搜索所有mixin以添加__table_args__,然后检查当前类以添加__local_table_args__.这样,__ local_table_args__不与声明的attr冲突.以相反的顺序检查基类(cls.mro()),以便链中较低的mixins被更高的mixin覆盖.

def _process_args(cls, attr, out_args, out_kwargs):
    try:
        args = getattr(cls, attr)
    except AttributeError:
        return

    if isinstance(args, Mapping):  # it's a dictionary
        out_kwargs.update(args)
    else:  # it's a list
        if isinstance(args[-1], Mapping):  # it has a dictionary at the end
            out_kwargs.update(args.pop())

        out_args.extend(args)

class Base():
    @declared_attr
    def __mapper_args__(cls):
        args = []
        kwargs = {}

        for mixin in reversed(cls.mro()):
            _process_args(mixin, '__mapper_args__', args, kwargs)

        _process_args(mixin, '__local_mapper_args__', args, kwargs)

        return kwargs  # mapper only takes dict

    @declared_attr
    def __table_args__(cls):
        args = []
        kwargs = {}

        for mixin in reversed(cls.mro()):
            _process_args(mixin, '__table_args__', args, kwargs)

        _process_args(cls, '__local_table_args__', args, kwargs)

        args.append(kwargs)  # [item, item, ...,  kwargs]
        return tuple(args)

所有mixin都应该正常定义__table_args__,但是从Base继承的“real”类应该定义__local_table_args__.