TMSS-778: test that we can set the error reason once, and are not allowed to overwrite it

SAS/TMSS/backend/test/t_subtasks.py

@@ -754,9 +754,17 @@ class SubtaskAllowedStateTransitionsTest(unittest.TestCase):
 
             # then go to the error state (should be allowed from any of these intermediate states)
             subtask.state = SubtaskState.objects.get(value=SubtaskState.Choices.ERROR.value)
-            subtask.error_reason = 'test_helper_method_set_subtask_state_following_allowed_transitions_error_path'
+            ERROR_MESSAGE = 'test_helper_method_set_subtask_state_following_allowed_transitions_error_path'
+            subtask.error_reason = ERROR_MESSAGE
             subtask.save()
             self.assertEqual(SubtaskState.Choices.ERROR.value, subtask.state.value)
+            self.assertEqual(ERROR_MESSAGE, subtask.error_reason)
+
+            # overwriting error reason should not be allowed
+            from django.db.utils import InternalError
+            with self.assertRaises(InternalError) as context:
+                subtask.error_reason = "overwriting error reason"
+                subtask.save()
 
     def test_helper_method_set_subtask_state_following_allowed_transitions_cancel_path(self):
         for desired_end_state_value in (SubtaskState.Choices.CANCELLING.value,SubtaskState.Choices.CANCELLED.value):