[sqlalchemy] generalized polymorphic mixin

[scott]

Is it possible to make a generalized declarative mixin class that
abstracts away all of the syntax of inheritance? I've seen examples
that set up the __mapper_args__ but not the discriminator column, and
examples that set up the discriminator column but not the
__mapper_args__, but none with both.

This is roughly how I imagine it should work, but when I tried this,
rows were created with null values for the discriminator. A full
example is here:
https://gist.github.com/797893

class PolymorphicMixin(object):
    @declared_attr
    def discriminator(cls):
        if Base in cls.__bases__:
            return Column('discriminator', types.String(50))
        for b in cls.__bases__:
            if hasattr(b, 'discriminator'):
                return b.discriminator

    @declared_attr
    def __mapper_args__(cls):
        ret = {'polymorphic_identity': cls.__name__}
        if Base in cls.__bases__:
            ret['polymorphic_on'] = PolymorphicMixin.discriminator
        return ret

Thanks,
Scott

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