id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
25139	ModelFormSet: allow swapping unique values	Jon Dufresne	Parth Patil	"As a user, I would expect to be able to swap ''unique'' values in an HTML form backed by a ModelFormSet. Turns out, this is not the case. The user must do a two step process of first setting the values to temporary value, then swapping the values. I have created a test case in the Django project that demonstrates my expectation in tests/model_formsets/tests.py:

{{{
    def test_swap_unique_values(self):
        p0 = Product.objects.create(slug='car-red')
        p1 = Product.objects.create(slug='car-blue')
        FormSet = modelformset_factory(Product, fields=""__all__"")
        data = {
            'form-TOTAL_FORMS': '2',
            'form-INITIAL_FORMS': '2',
            # Swap slug values.
            'form-0-id': p0.pk,
            'form-0-slug': 'car-blue',
            'form-1-id': p1.pk,
            'form-1-slug': 'car-red',
        }
        formset = FormSet(data)
        self.assertTrue(formset.is_valid())
        formset.save()

        p0 = Product.objects.get(pk=p0.pk)
        self.assertEquals(p0.slug, 'car-blue')
        p1 = Product.objects.get(pk=p1.pk)
        self.assertEquals(p1.slug, 'car-red')
}}}

However, the form fails to validate on the is_valid() line with the message ""Product with this Slug already exists."".

When thinking about the details, I understand why this happens. But I don't think it should error in this way. Swapping values is a convenience that a program can handle without the intervention of a human.

Knowing that in the end all values will be unique, I think the Django form engine should go ahead with the steps necessary to get there. I think it is reasonable for a user to expect that they could swap unique values through an HTML form in this way.

As a point of reference, the following test passes. It demonstrates a user using temporary values.

{{{
    def test_swap_unique_values_two_step(self):
        p0 = Product.objects.create(slug='car-red')
        p1 = Product.objects.create(slug='car-blue')
        FormSet = modelformset_factory(Product, fields=""__all__"")
        data = {
            'form-TOTAL_FORMS': '2',
            'form-INITIAL_FORMS': '2',
            'form-0-id': p0.pk,
            'form-0-slug': 'temp0',
            'form-1-id': p1.pk,
            'form-1-slug': 'temp1',
        }
        formset = FormSet(data)
        self.assertTrue(formset.is_valid())
        formset.save()

        data = {
            'form-TOTAL_FORMS': '2',
            'form-INITIAL_FORMS': '2',
            # Swap slug values.
            'form-0-id': p0.pk,
            'form-0-slug': 'car-blue',
            'form-1-id': p1.pk,
            'form-1-slug': 'car-red',
        }
        formset = FormSet(data)
        self.assertTrue(formset.is_valid())
        formset.save()

        p0 = Product.objects.get(pk=p0.pk)
        self.assertEquals(p0.slug, 'car-blue')
        p1 = Product.objects.get(pk=p1.pk)
        self.assertEquals(p1.slug, 'car-red')
}}}"	New feature	assigned	Forms	dev	Normal			johannes.linke@… Sergey Fedoseev Marco Beri	Someday/Maybe	0	0	0	0	0	0