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

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: 
>>>> @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 <> 
>>>>> 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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