[5.1.x] Fixed #35560 -- Made Model.full_clean() ignore GeneratedFields for constraints.

Accessing generated field values on unsaved models caused a crash when
validating CheckConstraints and UniqueConstraints with expressions.

Backport of 1005c2abd1 from main.

tests/model_fields/test_generatedfield.py

@@ -2,6 +2,7 @@ import uuid
 from decimal import Decimal
 
 from django.apps import apps
+from django.core.exceptions import ValidationError
 from django.db import IntegrityError, connection
 from django.db.models import (
     CharField,
@@ -18,6 +19,8 @@ from django.test.utils import isolate_apps
 from .models import (
     Foo,
     GeneratedModel,
+    GeneratedModelCheckConstraint,
+    GeneratedModelCheckConstraintVirtual,
     GeneratedModelFieldWithConverters,
     GeneratedModelNull,
     GeneratedModelNullVirtual,
@@ -25,6 +28,8 @@ from .models import (
     GeneratedModelOutputFieldDbCollationVirtual,
     GeneratedModelParams,
     GeneratedModelParamsVirtual,
+    GeneratedModelUniqueConstraint,
+    GeneratedModelUniqueConstraintVirtual,
     GeneratedModelVirtual,
 )
 
@@ -186,6 +191,42 @@ class GeneratedFieldTestMixin:
         m = self._refresh_if_needed(m)
         self.assertEqual(m.field, 3)
 
+    @skipUnlessDBFeature("supports_table_check_constraints")
+    def test_full_clean_with_check_constraint(self):
+        model_name = self.check_constraint_model._meta.verbose_name.capitalize()
+
+        m = self.check_constraint_model(a=2)
+        m.full_clean()
+        m.save()
+        m = self._refresh_if_needed(m)
+        self.assertEqual(m.a_squared, 4)
+
+        m = self.check_constraint_model(a=-1)
+        with self.assertRaises(ValidationError) as cm:
+            m.full_clean()
+        self.assertEqual(
+            cm.exception.message_dict,
+            {"__all__": [f"Constraint “{model_name} a > 0” is violated."]},
+        )
+
+    @skipUnlessDBFeature("supports_expression_indexes")
+    def test_full_clean_with_unique_constraint_expression(self):
+        model_name = self.unique_constraint_model._meta.verbose_name.capitalize()
+
+        m = self.unique_constraint_model(a=2)
+        m.full_clean()
+        m.save()
+        m = self._refresh_if_needed(m)
+        self.assertEqual(m.a_squared, 4)
+
+        m = self.unique_constraint_model(a=2)
+        with self.assertRaises(ValidationError) as cm:
+            m.full_clean()
+        self.assertEqual(
+            cm.exception.message_dict,
+            {"__all__": [f"Constraint “{model_name} a” is violated."]},
+        )
+
     def test_create(self):
         m = self.base_model.objects.create(a=1, b=2)
         m = self._refresh_if_needed(m)
@@ -305,6 +346,8 @@ class GeneratedFieldTestMixin:
 class StoredGeneratedFieldTests(GeneratedFieldTestMixin, TestCase):
     base_model = GeneratedModel
     nullable_model = GeneratedModelNull
+    check_constraint_model = GeneratedModelCheckConstraint
+    unique_constraint_model = GeneratedModelUniqueConstraint
     output_field_db_collation_model = GeneratedModelOutputFieldDbCollation
     params_model = GeneratedModelParams
 
@@ -318,5 +361,7 @@ class StoredGeneratedFieldTests(GeneratedFieldTestMixin, TestCase):
 class VirtualGeneratedFieldTests(GeneratedFieldTestMixin, TestCase):
     base_model = GeneratedModelVirtual
     nullable_model = GeneratedModelNullVirtual
+    check_constraint_model = GeneratedModelCheckConstraintVirtual
+    unique_constraint_model = GeneratedModelUniqueConstraintVirtual
     output_field_db_collation_model = GeneratedModelOutputFieldDbCollationVirtual
     params_model = GeneratedModelParamsVirtual