Fixed #25987 -- Made inline formset validation respect unique_together with an unsaved parent object.

Thanks Anton Kuzmichev for the report and Tim for the review.

django/forms/models.py

@@ -564,6 +564,9 @@ class BaseModelFormSet(BaseFormSet):
     """
     model = None
 
+    # Set of fields that must be unique among forms of this set.
+    unique_fields = set()
+
     def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
                  queryset=None, **kwargs):
         self.queryset = queryset
@@ -677,9 +680,11 @@ class BaseModelFormSet(BaseFormSet):
         for uclass, unique_check in all_unique_checks:
             seen_data = set()
             for form in valid_forms:
-                # get data for each field of each of unique_check
+                # Get the data for the set of fields that must be unique among the forms.
-                row_data = (form.cleaned_data[field]
+                row_data = (
-                            for field in unique_check if field in form.cleaned_data)
+                    field if field in self.unique_fields else form.cleaned_data[field]
+                    for field in unique_check if field in form.cleaned_data
+                )
                 # Reduce Model instances to their primary key values
                 row_data = tuple(d._get_pk_val() if hasattr(d, '_get_pk_val') else d
                                  for d in row_data)
@@ -878,6 +883,7 @@ class BaseInlineFormSet(BaseModelFormSet):
             qs = queryset.filter(**{self.fk.name: self.instance})
         else:
             qs = queryset.none()
+        self.unique_fields = {self.fk.name}
         super(BaseInlineFormSet, self).__init__(data, files, prefix=prefix,
                                                 queryset=qs, **kwargs)
 

tests/inline_formsets/models.py

@@ -31,5 +31,8 @@ class Poem(models.Model):
     poet = models.ForeignKey(Poet, models.CASCADE)
     name = models.CharField(max_length=100)
 
+    class Meta:
+        unique_together = ('poet', 'name')
+
     def __str__(self):
         return self.name

tests/inline_formsets/tests.py

@@ -162,3 +162,17 @@ class InlineFormsetFactoryTest(TestCase):
         PoemFormSet = inlineformset_factory(Poet, Poem, fields="__all__", extra=0)
         formset = PoemFormSet(None, instance=poet)
         self.assertEqual(len(formset.forms), 1)
+
+    def test_unsaved_fk_validate_unique(self):
+        poet = Poet(name='unsaved')
+        PoemFormSet = inlineformset_factory(Poet, Poem, fields=['name'])
+        data = {
+            'poem_set-TOTAL_FORMS': '2',
+            'poem_set-INITIAL_FORMS': '0',
+            'poem_set-MAX_NUM_FORMS': '2',
+            'poem_set-0-name': 'Poem',
+            'poem_set-1-name': 'Poem',
+        }
+        formset = PoemFormSet(data, instance=poet)
+        self.assertFalse(formset.is_valid())
+        self.assertEqual(formset.non_form_errors(), ['Please correct the duplicate data for name.'])