ModelForm fields with callable defaults don't correctly propagate default values

[Benjamin Rigaud]

When creating an object via the admin, if an inline contains an ArrayField in error, the validation will be bypassed (and the inline dismissed) if we submit the form a second time (without modification).

• go to /admin/my_app/thing/add/
• type anything in plop
• submit -> it shows an error on the inline
• submit again -> no errors, plop become unfilled

# models.py

class Thing(models.Model):
    pass


class RelatedModel(models.Model):
    thing = models.ForeignKey(Thing, on_delete=models.CASCADE)

    plop = ArrayField(
        models.CharField(max_length=42),
        default=list,
    )


# admin.py

class RelatedModelForm(forms.ModelForm):
    def clean(self):
        raise ValidationError("whatever")


class RelatedModelInline(admin.TabularInline):
    form = RelatedModelForm
    model = RelatedModel
    extra = 1


@admin.register(Thing)
class ThingAdmin(admin.ModelAdmin):
    inlines = [
        RelatedModelInline
    ]

It seems related to the hidden input containing the initial value:

<input type="hidden" name="initial-relatedmodel_set-0-plop" value="test" id="initial-relatedmodel_set-0-id_relatedmodel_set-0-plop">

I can fix the issue locally by forcing show_hidden_initial=False on the field (in the form init)

[Benjamin Rigaud]

First submit

[Benjamin Rigaud]

Second submit

[Mariusz Felisiak]

Can you reproduce this issue with Django 4.1? (or with the current main branch). Django 3.2 is in extended support so it doesn't receive bugfixes anymore (except security patches).

[Benjamin Rigaud]

Replying to Mariusz Felisiak:

Can you reproduce this issue with Django 4.1? (or with the current main branch). Django 3.2 is in extended support so it doesn't receive bugfixes anymore (except security patches).

Same issue with Django 4.1.2

[David Sanders]

This is because the inline form is considered to be changed on the first submission but not on the second submission.

Fields with callable defaults have a hidden widget which then used to detect if it's changed – a mechanism implemented long ago to preserve changing defaults between submissions like timezone.now.

I think the problem here is that the hidden widget is being populated with the submitted data instead of the result of the callable default, thus making the form think it's unchanged. I've submitted a PR to prevent these hidden widgets from being overridden by submitted values.

[David Sanders]

For reference here's the original ticket implementing the logic causing behaviour: #7975

[Mariusz Felisiak]

You shouldn't mark your own PRs as "Ready for Checkin".

[Mariusz Felisiak]

#33871 was a duplicate for JSONField.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 25904db9:

Fixed #34119 -- Prevented callable default hidden widget value from being overridden.

Thanks to Benjamin Rigaud for the report.

[GitHub <noreply@…>]

In 60a7bd8:

Refs #34119 -- Skipped test_callable_default_hidden_widget_value_not_overridden when JSONField is not supported.