On Apr 20, 2012, at 8:51 AM, lars van gemerden wrote:
> Ok, thank you, that helps, but now i cannot inherit from Engineer, as in:
>
> class BaseMixin(object):
>
> discriminator = Column(String(50))
>
> @declared_attr
> def __tablename__(cls):
> return cls.__name__
> @declared_attr
> def id(cls):
> return Column(Integer, primary_key = True)
> @declared_attr
> def __mapper_args__(cls):
> if not has_inherited_table(cls):
> return {'polymorphic_on': 'discriminator'}
> else:
> return {'polymorphic_identity': cls.__name__}
>
>
> class InheritMixin(BaseMixin):
> @declared_attr
> def id(cls):
> super_id = super(InheritMixin, cls).id
> return Column(Integer, ForeignKey(super_id), primary_key = True)
>
> class Person(BaseMixin, Base):
> name = Column(String(50))
>
> class Engineer(InheritMixin, Person):
> job = Column(String(50))
>
> class MasterEngineer(InheritMixin, Engineer):
> specialty = Column(String(50))
>
> Gives an MRO() error and if i would reverse the baseclasses (like class
> Engineer(Person, InheritMixin): ... ), the inheriting classes pick up the
> wrong id.
>
> Do you see any solution for this?
yeah I suppose if you're building out joined inheritance more than one level
then this becomes awkward. I never use joined inh more than one level because
it has too much of an impact on queries.
the metaclass as you mention is always the last resort when the various
declarative trickery reaches its limit. I'm not thrilled about the metaclass
approach because it quickly gets confusing and shouldn't be necessary. though
in this case without some extra mechanism on declarative, such as a
__pre_declare__() method of some kind, it might be the only approach.
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