[5.1.x] Fixed #35638 -- Updated validate_constraints to consider db_default.

Backport of 509763c79952cde02d9f5b584af4278bdbed77b2 from main.

10 files changed, 130 insertions, 13 deletions

diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index 7779173f3c..0f5e68b0d4 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -1250,9 +1250,41 @@ class Star(Expression):
 
 
 class DatabaseDefault(Expression):
-    """Placeholder expression for the database default in an insert query."""
+    """
+    Expression to use DEFAULT keyword during insert otherwise the underlying expression.
+    """
+
+    def __init__(self, expression, output_field=None):
+        super().__init__(output_field)
+        self.expression = expression
+
+    def get_source_expressions(self):
+        return [self.expression]
+
+    def set_source_expressions(self, exprs):
+        (self.expression,) = exprs
+
+    def resolve_expression(
+        self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False
+    ):
+        resolved_expression = self.expression.resolve_expression(
+            query=query,
+            allow_joins=allow_joins,
+            reuse=reuse,
+            summarize=summarize,
+            for_save=for_save,
+        )
+        # Defaults used outside an INSERT context should resolve to their
+        # underlying expression.
+        if not for_save:
+            return resolved_expression
+        return DatabaseDefault(
+            resolved_expression, output_field=self._output_field_or_none
+        )
 
     def as_sql(self, compiler, connection):
+        if not connection.features.supports_default_keyword_in_insert:
+            return compiler.compile(self.expression)
         return "DEFAULT", []
 
 
diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 796c4d23c4..d1f31f0211 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -983,13 +983,7 @@ class Field(RegisterLookupMixin):
 
     def pre_save(self, model_instance, add):
         """Return field's value just before saving."""
-        value = getattr(model_instance, self.attname)
-        if not connection.features.supports_default_keyword_in_insert:
-            from django.db.models.expressions import DatabaseDefault
-
-            if isinstance(value, DatabaseDefault):
-                return self._db_default_expression
-        return value
+        return getattr(model_instance, self.attname)
 
     def get_prep_value(self, value):
         """Perform preliminary non-db specific value checks and conversions."""
@@ -1031,7 +1025,9 @@ class Field(RegisterLookupMixin):
         if self.db_default is not NOT_PROVIDED:
             from django.db.models.expressions import DatabaseDefault
 
-            return DatabaseDefault
+            return lambda: DatabaseDefault(
+                self._db_default_expression, output_field=self
+            )
 
         if (
             not self.empty_strings_allowed
diff --git a/docs/releases/5.0.8.txt b/docs/releases/5.0.8.txt
index 31de9985c4..5cc3faec98 100644
--- a/docs/releases/5.0.8.txt
+++ b/docs/releases/5.0.8.txt
@@ -28,3 +28,7 @@ Bugfixes
 * Fixed a bug in Django 5.0 that caused a system check crash when
   ``ModelAdmin.date_hierarchy`` was a ``GeneratedField`` with an
   ``output_field`` of ``DateField`` or ``DateTimeField`` (:ticket:`35628`).
+
+* Fixed a bug in Django 5.0 which caused constraint validation to either crash
+  or incorrectly raise validation errors for constraints referring to fields
+  using ``Field.db_default`` (:ticket:`35638`).
diff --git a/tests/constraints/models.py b/tests/constraints/models.py
index 87b97b2a85..983d550502 100644
--- a/tests/constraints/models.py
+++ b/tests/constraints/models.py
@@ -128,3 +128,10 @@ class JSONFieldModel(models.Model):
 
     class Meta:
         required_db_features = {"supports_json_field"}
+
+
+class ModelWithDatabaseDefault(models.Model):
+    field = models.CharField(max_length=255)
+    field_with_db_default = models.CharField(
+        max_length=255, db_default=models.Value("field_with_db_default")
+    )
diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
index 32f98a5529..f54fb1faee 100644
--- a/tests/constraints/tests.py
+++ b/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 Abs, Lower
+from django.db.models.functions import Abs, Lower, Upper
 from django.db.transaction import atomic
 from django.test import SimpleTestCase, TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.test.utils import ignore_warnings
@@ -14,6 +14,7 @@ from .models import (
     ChildModel,
     ChildUniqueConstraintProduct,
     JSONFieldModel,
+    ModelWithDatabaseDefault,
     Product,
     UniqueConstraintConditionProduct,
     UniqueConstraintDeferrable,
@@ -396,6 +397,33 @@ class CheckConstraintTests(TestCase):
         with self.assertWarnsRegex(RemovedInDjango60Warning, msg):
             self.assertIs(constraint.check, other_condition)
 
+    def test_database_default(self):
+        models.CheckConstraint(
+            condition=models.Q(field_with_db_default="field_with_db_default"),
+            name="check_field_with_db_default",
+        ).validate(ModelWithDatabaseDefault, ModelWithDatabaseDefault())
+
+        # Ensure that a check also does not silently pass with either
+        # FieldError or DatabaseError when checking with a db_default.
+        with self.assertRaises(ValidationError):
+            models.CheckConstraint(
+                condition=models.Q(
+                    field_with_db_default="field_with_db_default", field="field"
+                ),
+                name="check_field_with_db_default_2",
+            ).validate(
+                ModelWithDatabaseDefault, ModelWithDatabaseDefault(field="not-field")
+            )
+
+        with self.assertRaises(ValidationError):
+            models.CheckConstraint(
+                condition=models.Q(field_with_db_default="field_with_db_default"),
+                name="check_field_with_db_default",
+            ).validate(
+                ModelWithDatabaseDefault,
+                ModelWithDatabaseDefault(field_with_db_default="other value"),
+            )
+
 
 class UniqueConstraintTests(TestCase):
     @classmethod
@@ -1251,3 +1279,30 @@ class UniqueConstraintTests(TestCase):
         msg = "A unique constraint must be named."
         with self.assertRaisesMessage(ValueError, msg):
             models.UniqueConstraint(fields=["field"])
+
+    def test_database_default(self):
+        models.UniqueConstraint(
+            fields=["field_with_db_default"], name="unique_field_with_db_default"
+        ).validate(ModelWithDatabaseDefault, ModelWithDatabaseDefault())
+        models.UniqueConstraint(
+            Upper("field_with_db_default"),
+            name="unique_field_with_db_default_expression",
+        ).validate(ModelWithDatabaseDefault, ModelWithDatabaseDefault())
+
+        ModelWithDatabaseDefault.objects.create()
+
+        msg = (
+            "Model with database default with this Field with db default already "
+            "exists."
+        )
+        with self.assertRaisesMessage(ValidationError, msg):
+            models.UniqueConstraint(
+                fields=["field_with_db_default"], name="unique_field_with_db_default"
+            ).validate(ModelWithDatabaseDefault, ModelWithDatabaseDefault())
+
+        msg = "Constraint “unique_field_with_db_default_expression” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            models.UniqueConstraint(
+                Upper("field_with_db_default"),
+                name="unique_field_with_db_default_expression",
+            ).validate(ModelWithDatabaseDefault, ModelWithDatabaseDefault())
diff --git a/tests/postgres_tests/migrations/0002_create_test_models.py b/tests/postgres_tests/migrations/0002_create_test_models.py
index 5538b436ad..188f79607d 100644
--- a/tests/postgres_tests/migrations/0002_create_test_models.py
+++ b/tests/postgres_tests/migrations/0002_create_test_models.py
@@ -434,7 +434,7 @@ class Migration(migrations.Migration):
                         primary_key=True,
                     ),
                 ),
-                ("ints", IntegerRangeField(null=True, blank=True)),
+                ("ints", IntegerRangeField(null=True, blank=True, db_default=(5, 10))),
                 ("bigints", BigIntegerRangeField(null=True, blank=True)),
                 ("decimals", DecimalRangeField(null=True, blank=True)),
                 ("timestamps", DateTimeRangeField(null=True, blank=True)),
diff --git a/tests/postgres_tests/models.py b/tests/postgres_tests/models.py
index a97894e327..e3118bc590 100644
--- a/tests/postgres_tests/models.py
+++ b/tests/postgres_tests/models.py
@@ -130,7 +130,7 @@ class LineSavedSearch(PostgreSQLModel):
 
 
 class RangesModel(PostgreSQLModel):
-    ints = IntegerRangeField(blank=True, null=True)
+    ints = IntegerRangeField(blank=True, null=True, db_default=(5, 10))
     bigints = BigIntegerRangeField(blank=True, null=True)
     decimals = DecimalRangeField(blank=True, null=True)
     timestamps = DateTimeRangeField(blank=True, null=True)
diff --git a/tests/postgres_tests/test_constraints.py b/tests/postgres_tests/test_constraints.py
index 3cc76cdcfe..9a572c9bf2 100644
--- a/tests/postgres_tests/test_constraints.py
+++ b/tests/postgres_tests/test_constraints.py
@@ -1238,3 +1238,12 @@ class ExclusionConstraintTests(PostgreSQLTestCase):
             constraint_name,
             self.get_constraints(ModelWithExclusionConstraint._meta.db_table),
         )
+
+    def test_database_default(self):
+        constraint = ExclusionConstraint(
+            name="ints_equal", expressions=[("ints", RangeOperators.EQUAL)]
+        )
+        RangesModel.objects.create()
+        msg = "Constraint “ints_equal” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            constraint.validate(RangesModel, RangesModel())
diff --git a/tests/validation/models.py b/tests/validation/models.py
index f6b1e0cd62..beec524758 100644
--- a/tests/validation/models.py
+++ b/tests/validation/models.py
@@ -48,7 +48,7 @@ class ModelToValidate(models.Model):
 
 class UniqueFieldsModel(models.Model):
     unique_charfield = models.CharField(max_length=100, unique=True)
-    unique_integerfield = models.IntegerField(unique=True)
+    unique_integerfield = models.IntegerField(unique=True, db_default=42)
     non_unique_field = models.IntegerField()
 
 
diff --git a/tests/validation/test_unique.py b/tests/validation/test_unique.py
index 4a8b3894f0..36ee6e9da0 100644
--- a/tests/validation/test_unique.py
+++ b/tests/validation/test_unique.py
@@ -146,6 +146,20 @@ class PerformUniqueChecksTest(TestCase):
             mtv = ModelToValidate(number=10, name="Some Name")
             mtv.full_clean()
 
+    def test_unique_db_default(self):
+        UniqueFieldsModel.objects.create(unique_charfield="foo", non_unique_field=42)
+        um = UniqueFieldsModel(unique_charfield="bar", non_unique_field=42)
+        with self.assertRaises(ValidationError) as cm:
+            um.full_clean()
+        self.assertEqual(
+            cm.exception.message_dict,
+            {
+                "unique_integerfield": [
+                    "Unique fields model with this Unique integerfield already exists."
+                ]
+            },
+        )
+
     def test_unique_for_date(self):
         Post.objects.create(
             title="Django 1.0 is released",