Refs #13091 -- Added test for commit=False idiom with partial unique_together validation.

tests/model_forms/tests.py

@@ -855,6 +855,34 @@ class UniqueTest(TestCase):
         self.assertEqual(len(form.errors), 1)
         self.assertEqual(form.errors['__all__'], ['Price with this Price and Quantity already exists.'])
 
+    def test_unique_together_exclusion(self):
+        """
+        Forms don't validate unique_together constraints when only part of the
+        constraint is included in the form's fields. This allows using
+        form.save(commit=False) and then assigning the missing field(s) to the
+        model instance.
+        """
+        class BookForm(forms.ModelForm):
+            class Meta:
+                model = DerivedBook
+                fields = ('isbn', 'suffix1')
+
+        # The unique_together is on suffix1/suffix2 but only suffix1 is part
+        # of the form. The fields must have defaults, otherwise they'll be
+        # skipped by other logic.
+        self.assertEqual(DerivedBook._meta.unique_together, (('suffix1', 'suffix2'),))
+        for name in ('suffix1', 'suffix2'):
+            with self.subTest(name=name):
+                field = DerivedBook._meta.get_field(name)
+                self.assertEqual(field.default, 0)
+
+        # The form fails validation with "Derived book with this Suffix1 and
+        # Suffix2 already exists." if the unique_together validation isn't
+        # skipped.
+        DerivedBook.objects.create(isbn='12345')
+        form = BookForm({'isbn': '56789', 'suffix1': '0'})
+        self.assertTrue(form.is_valid(), form.errors)
+
     def test_multiple_field_unique_together(self):
         """
         When the same field is involved in multiple unique_together