unique_together constraint is ignored in form generated from model, when inheritance comes into play

[Piotr Czachur]

Here you got test case that should pass, but it doesn't because unique_together constraint is ignored.
This happens when constraint is defined in parent class model, and there is child model extending that parent, and form is generated from child model.
Code is simple, just take a look if my explanation is messy.

from django.test import TestCase
from django.db import models
from django import forms
import unittest

class Person(models.Model):
    first_name = models.CharField(max_length=80)
    surname = models.CharField(max_length=80)

    class Meta:
        unique_together = (('first_name', 'surname'),)

class ExtPerson(Person):
    phone = models.CharField(max_length=80)


class PersonForm(forms.ModelForm):
    class Meta:
        model = Person

class ExtPersonForm(forms.ModelForm):
    class Meta:
        model = ExtPerson

class ExtPersonForm1(PersonForm):
    class Meta:
        model = ExtPerson


class TestUniqueTogetherValidation(TestCase):
    def setUp(self):
        Person(first_name='Chuck', surname='Norris').save()

    def tearDown(self):
        Person.objects.all().delete()

    # works as expected
    def test_unique_together_validation_should_work_for_person(self):
        data = {'first_name':'Chuck', 'surname':'Norris'}
        form = PersonForm(data)
        self.assertFalse(form.is_valid())

    # test will prove missing check for unique_together constrains from Person class   
    def test_unique_together_validation_should_work_for_extperson(self):
        data = {'first_name':'Chuck', 'surname':'Norris', 'phone':'1234'}
        form = ExtPersonForm(data)
        self.assertFalse(form.is_valid())

    # test will prove missing check for unique_together constrains from Person class   
    def test_unique_together_validation_should_work_for_extperson_when_model_form_inherits_from_PersonForm(self):
        data = {'first_name':'Chuck', 'surname':'Norris', 'phone':'1234'}
        form = ExtPersonForm1(data)
        self.assertFalse(form.is_valid())

    # test will prove missing check for unique_together constrains from Person class   
    def test_unique_together_validation_return_true_when_data_is_valid_so_writing_to_database_should_work_too(self):
        data = {'first_name':'Chuck', 'surname':'Norris', 'phone':'1234'}
        form = ExtPersonForm(data)
        self.assertTrue(form.is_valid())
        form.save()

Ran 4 tests in 0.219s
FAILED (failures=2, errors=1)

[Karen Tracey]

I believe this is #12881.

[Karen Tracey]

I'm wrong. The patch on #12881 does not fix this case, it appears the unique_together constraint is not considered for the child model. I suspect fixing this will require first fixing #12881 though.

[Karen Tracey]

New patch on #12881 attempts a comprehensive fix of inherited unique constraints, so this will be fixed by #12881 even though the original problem report is a bit different.