concrete inheritance is very challenging overall, if you expect there to be any
kind of polymorphic interaction between the classes. if you want to query
polymorphically then speed will be probably worse. If you can do without
polymorphic and stick to each subclass directly it wont have an issue.
Usually though if I'm inheriting more than two levels, I'll use joined
inheritance for level one->two then single table for all levels beyond that.
Depends on what you're trying to do.
On Apr 27, 2012, at 3:05 AM, lars van gemerden wrote:
> 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.
>
>
>
>
>
>
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