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).
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?
Ticket: https://code.djangoproject.com/ticket/27039
[1] https://github.com/django/django/pull/7068
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