Re: Model form with optional fields issue

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Hi Marcin,

On 12/19/2014 11:44 AM, marcin.j.nowak@gmail.com wrote:
> Hello!
>
> I have model form defined with all optional fields (required=False).
> I want use this form to update only provided fields leaving others
> unchanged.
> But Django replaces optional fields to empty values and removes data from
> models.
>
>
> A short example:
>
> class MyForm(forms.ModelForm):
> class Meta:
> model = User
> fields = ('username', 'email')
>
> def __init__(self, *args, **kw):
> super(MyForm, self).__init__(*args, **kw)
> for field in self.fields.values():
> field.required = False
>
>
> In [21]: admin = User.objects.get(username='admin')
>
> *In [22]: admin.email*
> *Out[22]: u'admin@example.com'*
>
> In [23]: form = MyForm(data={'username': 'admin'}, instance=admin)
>
> In [24]: form.is_valid()
> Out[24]: True
>
> In [25]: form.save()
> Out[25]: <User: admin>
>
> In [26]: admin = User.objects.get(username='admin')
>
> *In [27]: admin.email*
> *Out[27]: u''*
>
> In my opinion this is big, fat, ugly bug or a design misconception.
> If something is not provided does not mean that someting does not exist!
> Django should skip updating these unprovided fields by removig them from
> cleaned_data dict.
>
> You can even try to save this form without data (ok with data, but with
> unknown keys):
>
> form = MyForm(data={'unknownfield': 'some-value'}, instance=admin)
> form.is_valid()
>
> # form.cleaned_data has empty strings
>
> form.save()
>
>
> Username and email will be replaced with empty strings.
> Where is admin?, you'll ask... And you'll hear a strange voice - "admin is
> cooking borscht with ravioli, dude!"
>
> What do you think - is this expected?

Yes, this is expected. Django's form system is designed for use with
HTML forms, which do not provide partial data: unchanged field values
are sent too, so this problem does not occur.

That in itself wouldn't prevent switching to the behavior you want,
except that it is not compatible with how checkboxes are handled in HTML
forms. If a checkbox is unchecked, the browser will not send that
checkbox's name in the submitted POST data at all. So with the behavior
you're asking for, it would be impossible to ever change a boolean field
represented by a checkbox from True to False, using an HTML form.

Carl

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