Abstract model managers should return subclass instances

[Jeremy Dunck]

Suppose a base:

class Document(Model):
   title = CharField()
   
   class Meta:
     abstract = True

class Presentation(Document):
   num_kitten_pictures = IntegerField()

class Spreadsheet(Document):
    num_pie_charts = IntegerField()

It would be useful if Documents.objects.filter(title='x') returned instances of both Presentation and Spreadsheet whose title was 'x'.

I'd expect support only for base-class fields. This should be safe, since inheritance should follow an is-a relationship.

[mmcnickle]

Related discussion: ​http://groups.google.com/group/django-developers/browse_thread/thread/421d91a82ef474df

[Jacob]

Hm. This is starting to smell suspiciously like "narrow" support, which is a whole nest of works. Jeremy, would you expect to only support this for ABCs? Or would you want to see concrete base class support as well?

[anonymous]

Well, I just meant for ABCs, but I see that this is a slippery slope. I think in general, concrete inheritance is to be avoided, but I understand it may be useful at times.

I suppose part of the problem is that a concrete base may be inherited by multiple subclasses so that there's no right answer in what instance to return. Diamond inheritance out of the ORM makes me queasy.

I just feel that from a reusable app perspective (and encouraging reusable apps to ship ABCs in general) it would help the app writer quite a bit to be able to rely on instance(ABC) coming out of the ORM. Is there some way we could draw a bright line around concrete to avoid narrow/widen?

[Jeremy Dunck]

(That was me, jdunck)

[anonymous]

would there be a way to use two managers for the parent model?
i.e., parent.objects and parent.subclass_objects

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

On one hand, this is a cool feature, and on the other hand, it's a partial implementation of STI. It has been implemented externally, see django_polymorphic.

My gut feeling is that a partial implementation will be buggy in many edge cases. To include something like this in core, we must be confident that we can handle all cases correctly, and I don't think we have enough info yet.