结合使用SQLAlchemy中mixin类的约束

Question

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__。在

Combining Table/Mapper Arguments from Multiple Mixins

我的问题是，这个例子展示了如何在链的末尾（MRO中的最后一个类）实现这一点，但是如果我有这些mixin并希望它出现在MyClientMixin或{}类中，以避免为其他类型的mixin重复这段代码，那么如何实现这一点呢？在

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

我对这个概念有点纠结。在

Answer 1

这个基类将在所有mixin中搜索要添加的__table_args__，然后检查当前类中是否有要添加的__local_table_args__。这样，__local_table_args__就不会与声明的attr冲突。基类（cls.mro()）按相反的顺序进行检查，以便链的较低的mixin被更高的mixin覆盖。在

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)

所有的mixin都应该像正常一样定义__table_args__，但是继承自Base的“real”类应该定义__local_table_args__。在