[5.1.x] Refs #30581 -- Made unattached UniqueConstraint(fields) validation testable.

The logic allowing UniqueConstraint(fields).validate to preserve backward
compatiblity with Model.unique_error_message failed to account for cases where
the constraint might not be attached to a model which is a common pattern
during testing.

This changes allows for arbitrary UniqueConstraint(fields) to be tested in
isolation without requiring actual models backing them up.

Co-authored-by: Mark G <mark.gensler@protonmail.com>

Backport of 13922580cccfb9ab2922ff4943dd39da56dfbd8c from main.

2 files changed, 16 insertions, 12 deletions

diff --git a/django/db/models/constraints.py b/django/db/models/constraints.py
index 3e6c5205c6..9bb407274c 100644
--- a/django/db/models/constraints.py
+++ b/django/db/models/constraints.py
@@ -653,19 +653,16 @@ class UniqueConstraint(BaseConstraint):
             queryset = queryset.exclude(pk=model_class_pk)
         if not self.condition:
             if queryset.exists():
-                if self.expressions:
+                if self.fields:
+                    # When fields are defined, use the unique_error_message() for
+                    # backward compatibility.
                     raise ValidationError(
-                        self.get_violation_error_message(),
-                        code=self.violation_error_code,
+                        instance.unique_error_message(model, self.fields),
                     )
-                # When fields are defined, use the unique_error_message() for
-                # backward compatibility.
-                for model, constraints in instance.get_constraints():
-                    for constraint in constraints:
-                        if constraint is self:
-                            raise ValidationError(
-                                instance.unique_error_message(model, self.fields),
-                            )
+                raise ValidationError(
+                    self.get_violation_error_message(),
+                    code=self.violation_error_code,
+                )
         else:
             against = instance._get_field_value_map(meta=model._meta, exclude=exclude)
             try:
diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
index 86efaa79e7..8b7599adc1 100644
--- a/tests/constraints/tests.py
+++ b/tests/constraints/tests.py
@@ -896,6 +896,13 @@ class UniqueConstraintTests(TestCase):
                 ChildUniqueConstraintProduct(name=self.p1.name, color=self.p1.color),
             )
 
+    def test_validate_fields_unattached(self):
+        Product.objects.create(price=42)
+        constraint = models.UniqueConstraint(fields=["price"], name="uniq_prices")
+        msg = "Product with this Price already exists."
+        with self.assertRaisesMessage(ValidationError, msg):
+            constraint.validate(Product, Product(price=42))
+
     @skipUnlessDBFeature("supports_partial_indexes")
     def test_validate_condition(self):
         p1 = UniqueConstraintConditionProduct.objects.create(name="p1")
@@ -921,7 +928,7 @@ class UniqueConstraintTests(TestCase):
         )
 
     @skipUnlessDBFeature("supports_partial_indexes")
-    def test_validate_conditon_custom_error(self):
+    def test_validate_condition_custom_error(self):
         p1 = UniqueConstraintConditionProduct.objects.create(name="p1")
         constraint = models.UniqueConstraint(
             fields=["name"],