[5.0.x] Refs #34936 -- Added test for altering DecimalField with db_default to non-nullable.

Backport of 1944f490f91e44e1378d53420b64a78163aea815 from main

tests/migrations/test_operations.py

@@ -1,4 +1,5 @@
 import math
+from decimal import Decimal
 
 from django.core.exceptions import FieldDoesNotExist
 from django.db import IntegrityError, connection, migrations, models, transaction
@@ -2219,6 +2220,43 @@ class OperationTests(OperationTestBase):
         pony = project_state.apps.get_model(app_label, "pony").objects.create(weight=1)
         self.assertIsNone(pony.green)
 
+    def test_alter_field_change_nullable_to_decimal_database_default_not_null(self):
+        app_label = "test_alflcntdddn"
+        project_state = self.set_up_test_model(app_label)
+        operation_1 = migrations.AddField(
+            "Pony",
+            "height",
+            models.DecimalField(null=True, max_digits=5, decimal_places=2),
+        )
+        operation_2 = migrations.AlterField(
+            "Pony",
+            "height",
+            models.DecimalField(
+                max_digits=5, decimal_places=2, db_default=Decimal("12.22")
+            ),
+        )
+        table_name = f"{app_label}_pony"
+        self.assertColumnNotExists(table_name, "height")
+        # Add field.
+        new_state = project_state.clone()
+        operation_1.state_forwards(app_label, new_state)
+        with connection.schema_editor() as editor:
+            operation_1.database_forwards(app_label, editor, project_state, new_state)
+        self.assertColumnExists(table_name, "height")
+        old_pony = new_state.apps.get_model(app_label, "pony").objects.create(weight=1)
+        self.assertIsNone(old_pony.height)
+        # Alter field.
+        project_state, new_state = new_state, new_state.clone()
+        operation_2.state_forwards(app_label, new_state)
+        with connection.schema_editor() as editor:
+            operation_2.database_forwards(app_label, editor, project_state, new_state)
+        old_pony.refresh_from_db()
+        self.assertEqual(old_pony.height, Decimal("12.22"))
+        pony = new_state.apps.get_model(app_label, "pony").objects.create(weight=2)
+        if not connection.features.can_return_columns_from_insert:
+            pony.refresh_from_db()
+        self.assertEqual(pony.height, Decimal("12.22"))
+
     @skipIfDBFeature("interprets_empty_strings_as_nulls")
     def test_alter_field_change_blank_nullable_database_default_to_not_null(self):
         app_label = "test_alflcbnddnn"