Ok, so speed might become an issue for me as well;
Do you think a similar metaclass approach would work for concrete
inheritance would work without major drawbacks (before i do a major
overhaul)?
Is there any indication about how much faster concrete inheritance is,
compared to joined inheritance?
Cheers, Lars
On Friday, April 20, 2012 9:32:49 PM UTC+2, Michael Bayer wrote:
>
>
> 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.
>
>
--
You received this message because you are subscribed to the Google Groups
"sqlalchemy" group.
To view this discussion on the web visit
https://groups.google.com/d/msg/sqlalchemy/-/MH4tZazKT0EJ.
To post to this group, send email to [email protected].
To unsubscribe from this group, send email to
[email protected].
For more options, visit this group at
http://groups.google.com/group/sqlalchemy?hl=en.
