That's what we've done in Django 1.7 for Form/ModelForm (#19617, #8620), and I completely agree that it should be possible to shadow fields from abstract models. The docs even suggest that this may be possible in the future; quoting the relevant section: "this [shadowing] is not permitted for attributes that are Field instances (at least, not at the moment)."
For consistency with forms - and because we've put a great deal of thinking into it - the implementation should be: you can shadow a field with another field, or you can remove a field using None as a sentinel value (see #22510 for more details). I believe we have a safety net in the form of migrations, if you accidentally shadow a field, migrations will pick it up, so it's unlikely to go unnoticed. I'd be quite happy to shepherd this patch. Unit tests should cover those scenarios: - An abstract model redefines or removes some fields from a parent abstract model. - A concrete model redefines or removes some fields from a parent abstract model. - Ensure that the standard MRO resolution applies when you do Concrete(AbstractA, AbstractB) with AbstractA redefining a field from AbstractB. The last case may be tricky because if I remember well ModelBase iterates through the MRO in the wrong direction (i.e. bases should be iterated over in reverse). We also need some tests to show how migrations behave. -- Loïc > On Feb 7, 2015, at 09:33, Marten Kenbeek <[email protected]> wrote: > > Hi Russ, > > I can see your point on accidentally overriding fields, though I'm not sure I > agree. In any other case, such as normal attributes and methods, there is no > safety net for overriding attributes either. Any model that would be affected > by this change would also raise an error on import without the patch, so > existing functional code won't be affected. On the other hand, a new > parameter for the field wouldn't be that much of a hassle to implement or > work with. I'd be interested to hear what others think about this. > > There are more than a few questions on stack overflow that expect this > behaviour, even if the docs specifically mention that it won't work. If users > intuitively try to override fields in this manner, I think it would be > reasonable to allow this without any explicit arguments. > > We can always restrict what you can override, at least as the default > behaviour, e.g. so that you can only use subclasses of the original field. > That would make code that depends on the abstract field less prone to bugs, > though subtle bugs can still happen if you e.g. override the max length of a > field. > > This was indeed just a proof-of-concept. That remark was meant more like "it > doesn't appear to break everything". > > Marten > > Op vrijdag 6 februari 2015 23:48:55 UTC+1 schreef Marten Kenbeek: > Hey, > > While fiddling with django I noticed it would be trivial to add support for > shadowing fields from abstract base models. As abstract fields don't exist in > the database, you simply have to remove the check that reports name clashes > for abstract models, and no longer override the field. > > This would allow you to both override fields from third-party abstract models > (e.g. change the username length of AbstractUser), and to override fields of > common abstract models on a per-model basis. Previously you'd have to copy > the original abstract model just to change a single field, or alter the field > after the model definition. The first approach violates the DRY principle, > the second approach can introduce subtle bugs if the field is somehow used > before you changed it (rare, but it can happen). > > This patch adds the feature. It seems to work correctly when using the models > and creating/applying migrations, and passes the complete test suite. > > What do you guys think about this feature? > > -- > You received this message because you are subscribed to the Google Groups > "Django developers (Contributions to Django itself)" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > Visit this group at http://groups.google.com/group/django-developers. > To view this discussion on the web visit > https://groups.google.com/d/msgid/django-developers/072149e1-c794-446d-957b-f5fc5df87096%40googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Django developers (Contributions to Django itself)" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/django-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/django-developers/37941396-B9E2-4C5B-9834-03C299DAC2AB%40gmail.com. For more options, visit https://groups.google.com/d/optout.
