[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.

1 files changed, 16 insertions, 0 deletions

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.