Model form with optional fields issue

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? 

/Marcin

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