[1.8.x] Fixed #24595 -- Prevented loss of null info in MySQL field alteration

Thanks Simon Percivall for the report, and Simon Charette and Tim
Graham for the reviews.
Backport of 02260ea3f6 from master.

2 files changed, 27 insertions, 2 deletions

diff --git a/tests/migrations/test_operations.py b/tests/migrations/test_operations.py
index a351154058..7ff798f690 100644
--- a/tests/migrations/test_operations.py
+++ b/tests/migrations/test_operations.py
@@ -1101,9 +1101,18 @@ class OperationTests(OperationTestBase):
 
         def assertIdTypeEqualsFkType():
             with connection.cursor() as cursor:
-                id_type = [c.type_code for c in connection.introspection.get_table_description(cursor, "test_alflpkfk_pony") if c.name == "id"][0]
-                fk_type = [c.type_code for c in connection.introspection.get_table_description(cursor, "test_alflpkfk_rider") if c.name == "pony_id"][0]
+                id_type, id_null = [
+                    (c.type_code, c.null_ok)
+                    for c in connection.introspection.get_table_description(cursor, "test_alflpkfk_pony")
+                    if c.name == "id"
+                ][0]
+                fk_type, fk_null = [
+                    (c.type_code, c.null_ok)
+                    for c in connection.introspection.get_table_description(cursor, "test_alflpkfk_rider")
+                    if c.name == "pony_id"
+                ][0]
             self.assertEqual(id_type, fk_type)
+            self.assertEqual(id_null, fk_null)
 
         assertIdTypeEqualsFkType()
         # Test the database alteration
diff --git a/tests/schema/tests.py b/tests/schema/tests.py
index 2f4a84db09..7027d716b0 100644
--- a/tests/schema/tests.py
+++ b/tests/schema/tests.py
@@ -426,6 +426,22 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_field(Note, old_field, new_field, strict=True)
 
+    def test_alter_field_keep_null_status(self):
+        """
+        Changing a field type shouldn't affect the not null status.
+        """
+        with connection.schema_editor() as editor:
+            editor.create_model(Note)
+        with self.assertRaises(IntegrityError):
+            Note.objects.create(info=None)
+        old_field = Note._meta.get_field("info")
+        new_field = CharField(max_length=50)
+        new_field.set_attributes_from_name("info")
+        with connection.schema_editor() as editor:
+            editor.alter_field(Note, old_field, new_field, strict=True)
+        with self.assertRaises(IntegrityError):
+            Note.objects.create(info=None)
+
     def test_alter_null_to_not_null(self):
         """
         #23609 - Tests handling of default values when altering from NULL to NOT NULL.