Django 1.10 onwards broke previous behaviour for models.DateTimeField() in Admin

[Kegan Gan]

In Django 1.9 and prior, we can use a date widget in Admin for a datetime field. Specifically, we can override the default datetime widget in Admin using AdminDateWidget. In this way, user are only require to enter a date. The time will be default 0h 0m 0s.

From Django 1.10 onwards, this behaviour has break. Now, there is an error Enter a list of values. on save in Admin.

Example code.

Model:

class Highlight(models.Model):
    end_at = models.DateTimeField()

    def save(self, *args, **kwargs):
        # Add 23 hours, 59 minutes, 59 seconds to end_at so that it represent end of day.
        extratime = timedelta(hours=23, minutes=59, seconds=59)
        if self.end_at:
            self.end_at = self.end_at + extratime
        super(Highlight, self).save(*args, **kwargs)

Admin:

class HighlightAdmin(admin.ModelAdmin):
    formfield_overrides = {
        models.DateTimeField: {'widget': widgets.AdminDateWidget},
    }

Now there is no easy way to let user only input the date part of datetimefield in Admin.

[Tim Graham]

Can you ​bisect to find where the behavior changed? Without looking into the details, I'm not immediately convinced we should treat this as a bug and provide backwards-compatibility for it, but we'll see.

[Tim Graham]

Bisected to b9290b1d49538c1092b59c41e6046b11c25ecafb. To adapt your code, you must add 'form_class': forms.DateField to the override.

[Kegan Gan]

Thanks Tim! It worked.

For reference, to override admin widget, use the following:

    formfield_overrides = {
        models.DateTimeField: {'form_class': forms.DateTimeField, 'widget': widgets.AdminDateWidget},
    }

Also refer to #26449.