On Fri, Aug 12, 2016 at 9:58 AM, Tim Graham <[email protected]> wrote:
> A change in Django 1.10 inadvertently caused the following behavior change:
>
> class M(models.Model):
> f = models.CharField(max_length=255, default='default_value')
>
> class MF(forms.ModelForm):
> f = forms.CharField(required=False)
>
> class Meta:
> model = M
> fields = ['f']
>
> >>> mf = MF({})
> >>> m = mf.save()
> >>> m.f
> '' (Django 1.10+)
> 'default_value' (Django < 1.10)
>
>
> On Django < 1.10, {'f': ''} as the form data (rather than empty form
> data) also results in a model instance with the model field default rather
> than an empty string. Claude and I agree that for this latter case,
> transforming an empty POST value to a model field default doesn't seem
> correct (that is, the new behavior seems better).
>
That certainly sounds incorrect to me and I would not want to see pre-1.10
behavior restored for the blank ('') value case.
> However, the desired behavior in the empty POST data case isn't so clear.
> For example, I could imagine an API call where it would be useful for the
> model to use the default if a user doesn't submit any data for that field.
> An issue with the idea that values not present in POST data fallback to
> their default is for HTML widgets like checkboxes. If a checkbox isn't
> checked, the key won't appear in POST data. In such a case, it's
> inappropriate for an unchecked checkbox to fallback to the model field's
> default if it's True. A draft patch to restore the Django < 1.10 behavior
> [1] special cases this with isinstance(form[f.name].field.widget,
> CheckboxInput) to fix some test failures but that hardly seems ideal.
>
> Do you feel we should try to restore the Django < 1.10 behavior or keep
> the new behavior and document that model field defaults are used to
> populate initial blank forms but not to fill missing data from the form
> input?
>
I tend to prefer the more cautious approach, i.e., allow the application to
supply default values to the form if they're not included in the POST, but
it sounds like I might be in the minority.
Tobias
--
*Tobias McNulty*Chief Executive Officer
[email protected]
www.caktusgroup.com
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