[5.1.x] Fixed #35594 -- Added unique nulls distinct validation for expressions.

Thanks Mark Gensler for the report.

Backport of adc0b6aac3 from main.

django/db/models/constraints.py

@@ -8,7 +8,7 @@ from django.db import connections
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.expressions import Exists, ExpressionList, F, RawSQL
 from django.db.models.indexes import IndexExpression
-from django.db.models.lookups import Exact
+from django.db.models.lookups import Exact, IsNull
 from django.db.models.query_utils import Q
 from django.db.models.sql.query import Query
 from django.db.utils import DEFAULT_DB_ALIAS
@@ -642,12 +642,16 @@ class UniqueConstraint(BaseConstraint):
                     meta=model._meta, exclude=exclude
                 ).items()
             }
-            expressions = []
+            filters = []
             for expr in self.expressions:
                 if hasattr(expr, "get_expression_for_validation"):
                     expr = expr.get_expression_for_validation()
-                expressions.append(Exact(expr, expr.replace_expressions(replacements)))
-            queryset = queryset.filter(*expressions)
+                rhs = expr.replace_expressions(replacements)
+                condition = Exact(expr, rhs)
+                if self.nulls_distinct is False:
+                    condition = Q(condition) | Q(IsNull(expr, True), IsNull(rhs, True))
+                filters.append(condition)
+            queryset = queryset.filter(*filters)
         model_class_pk = instance._get_pk_val(model._meta)
         if not instance._state.adding and model_class_pk is not None:
             queryset = queryset.exclude(pk=model_class_pk)

docs/releases/5.0.8.txt

@@ -9,4 +9,5 @@ Django 5.0.8 fixes several bugs in 5.0.7.
 Bugfixes
 ========
 
-* ...
+* Added missing validation for ``UniqueConstraint(nulls_distinct=False)`` when
+  using ``*expressions`` (:ticket:`35594`).

tests/constraints/tests.py

@@ -4,7 +4,7 @@ from django.core.exceptions import ValidationError
 from django.db import IntegrityError, connection, models
 from django.db.models import F
 from django.db.models.constraints import BaseConstraint, UniqueConstraint
-from django.db.models.functions import Lower
+from django.db.models.functions import Abs, Lower
 from django.db.transaction import atomic
 from django.test import SimpleTestCase, TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.test.utils import ignore_warnings
@@ -1070,6 +1070,19 @@ class UniqueConstraintTests(TestCase):
         is_not_null_constraint.validate(Product, Product(price=4, discounted_price=3))
         is_not_null_constraint.validate(Product, Product(price=2, discounted_price=1))
 
+    def test_validate_nulls_distinct_expressions(self):
+        Product.objects.create(price=42)
+        constraint = models.UniqueConstraint(
+            Abs("price"),
+            nulls_distinct=False,
+            name="uniq_prices_nulls_distinct",
+        )
+        constraint.validate(Product, Product(price=None))
+        Product.objects.create(price=None)
+        msg = f"Constraint “{constraint.name}” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            constraint.validate(Product, Product(price=None))
+
     def test_name(self):
         constraints = get_constraints(UniqueConstraintProduct._meta.db_table)
         expected_name = "name_color_uniq"