Current status:
- Models in general seem to be working.
- Trying to override a concrete (or explicitly locked) field with an
abstract field will keep the concrete field, but not raise any errors. This
occurs when inheriting from (AbstractBase, ConcreteBase).
- I might still change this to raise an error, it migth cause a few
headaches when fields don't appear while they're first in the mro.
- Trying to override field.attname (e.g. foo_id) will fail checks unless
you explicitly set foo = None.
- Trying to override an abstract field with a reverse relation will fail
checks unless you explicitly set foo = None.
What is still needed:
- ? More extensive model tests. During development a few more bugs
popped up. I've written tests for them, but there might be some more.
- Explicit migration tests. Existing migration tests pass, but don't
include the new scenario's.
- Documentation.
My WIP branch can be found at
https://github.com/knbk/django/compare/ticket_24305.
Op dinsdag 10 februari 2015 14:36:53 UTC+1 schreef Marten Kenbeek:
>
> Hi Aron,
>
> With option 3, this would work:
>
> class Cousin(models.Model):
> child = models.ForeignKey(Child, allow_override=True)
>
> but it would raise an error without `allow_override`.
>
> I think my question really is: should we treat reverse relations as
> first-class fields, or not? If we do, 1) would be the logical choice but
> can cause problems. 3) would deal with those problems in a way and still
> allow a relation field from another model to override a local field. If we
> don't, that kind of implies 2). Removing the field with `None` would
> implicitly allow a reverse relation to take its place, but that reverse
> relation in and of itself would not be allowed to override any abstract
> fields.
>
> This is just an example. In real code this would more likely happen with
> an explicit `related_name`. Renaming the related name and living with an
> unused `cousin_set` attribute is the current solution, but with this
> feature proposal it would be nice to also have a way to override the
> inherited field and use it for a reverse relation instead.
>
>
> Op dinsdag 10 februari 2015 02:28:58 UTC+1 schreef Aron Podrigal:
>>
>> Why should this be treated differently than the general behavior when
>> realted_names clash when you have more than one foreign key to the same
>> model? So as one would normally do
>> set the related_name explicitly to some other value.
>> setting the field to None is just the way of removing a field and has
>> nothing more special related to the auto created reverse field descriptor.
>> about option 3 I didn't quite understand. can you explain a bit more?
>>
>> On Monday, February 9, 2015 at 4:25:22 PM UTC-5, Marten Kenbeek wrote:
>>>
>>> I'd like some additional opinions on the following:
>>>
>>> Say you have the following classes:
>>>
>>> class Parent(models.Model):
>>> cousin_set = models.CharField(max_length=100)
>>>
>>> class Meta:
>>> abstract = True
>>>
>>> class Child(Parent):
>>> pass
>>>
>>> class Cousin(models.Model):
>>> child = models.ForeignKey(Child)
>>>
>>> Obviously, the `cousin_set` field inherited from `Parent` clashes with
>>> the reverse relation from `Cousin`.
>>>
>>> I can see the following options:
>>> 1) Allow the reverse object descriptor to overwrite the field.
>>> 2) Only allow the reverse object descriptor to overwrite the field if it
>>> is explicitly set to None on the target model.
>>> 3) Only allow the reverse object descriptor to overwrite the field if
>>> the foreignkey/m2m/o2o field itself has a flag set to explicitly override.
>>>
>>> 1) is consistent with the behaviour of local fields, but I think it will
>>> be problematic if *other *models can silently overwrite a field. 3)
>>> would still allow other models to affect local fields, but at least it has
>>> a fail-safe that prevents you from accidentally overriding fields. 2) would
>>> only allow the inheriting model itself to change which fields it inherits.
>>>
>>> Problems caused by option 1) would be hard to debug when you don't know
>>> which code overrides your field, so I wouldn't do that. I think 2) would be
>>> the cleanest and most consistent way. Only local fields would override
>>> parent fields, but the sentinel value `None` would remove the field and
>>> free the name for reverse relations. I can also see the advantage of 3)
>>> over 2) when you don't have access to the model on the other side. However,
>>> I don't know enough about foreign key internals to know if 3) is at all
>>> feasible. What happens e.g. when only the target is loaded in a migration?
>>> Would it pick up that the remote foreign key overrides a local field? As
>>> adding reverse relations is a lazy, or at least delayed operation afaik,
>>> would it still be save to rely on that to override fields?
>>>
>>> I believe my current plans for the patch would automatically create
>>> situation 2 without any extra work. The field would no longer exist on the
>>> child class when the reverse relation is added. Option 3) would require an
>>> additional patch to the code that adds the reverse relationship, but it
>>> allows for some extra options.
>>>
>>> Any input? Additional options are also welcome.
>>>
>>> Op zondag 8 februari 2015 21:09:41 UTC+1 schreef Marten Kenbeek:
>>>>
>>>> The general reaction seems to be a yes if we can work out the kinks, so
>>>> I went ahead and created a ticket:
>>>> https://code.djangoproject.com/ticket/24305
>>>>
>>>> @Aron That's a good question. One option would be to lock certain
>>>> fields, so that they can't be changed if they are an integral part of the
>>>> model. That would be a simple solution, but that won't help for existing
>>>> code that doesn't lock the fields. It won't break existing code, but it
>>>> won't protect for errors either. The opt-in version (i.e. an 'unlock'
>>>> attribute) would lock many fields which would otherwise be completely safe
>>>> to overwrite.
>>>>
>>>> Another option would be more elaborate "requirements" for a manager or
>>>> some methods, i.e. allow the manager to specify the necessary class of a
>>>> certain field or a minimum length. If the modeldoesn't meet the
>>>> requirements, the manager or some of the methods will not be inherited.
>>>> While it allows for more control, this option would greatly increase the
>>>> complexity of the patch and requires more from the developers of custom
>>>> managers. It can also cause issues when the requirements aren't up-to-date
>>>> with the manager's methods.
>>>>
>>>> We could also say that it is the users responsibility and don't provide
>>>> special protection, in line with the fields themselves, but I guess that
>>>> this would generally be more problematic for custom managers. It can also
>>>> cause silent bugs when the manager's methods don't work as intended but
>>>> won't raise an exception either, which is not a good idea imho.
>>>>
>>>> I think the locking approach would be the easiest and most pragmatic
>>>> method. I think it's still - in part - the users responsibility to confirm
>>>> that a field can be overridden. The Django documentation could, where
>>>> applicable, document the requirements on fields that can be overridden,
>>>> i.e. that an AbstractUser's username must be a CharField (which isn't
>>>> necessarily true, just an example).
>>>>
>>>> @Loïc The bases are indeed traversed in the reversed order.
>>>>
>>>> Op zondag 8 februari 2015 08:19:57 UTC+1 schreef Loïc Bistuer:
>>>>>
>>>>> 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 <marte...@gmail.com>
>>>>> 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?
>>>>> >
>>>>> > --
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>>>>>
>>>>>
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>>>>>
>>>>>
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