diff --git a/src/Model/Item.php b/src/Model/Item.php
index 1f0d496ab8..7c4a3cbffe 100644
--- a/src/Model/Item.php
+++ b/src/Model/Item.php
@@ -78,6 +78,8 @@ class Item extends BaseObject
 			'author-id', 'author-link', 'author-name', 'author-avatar',
 			'owner-id', 'owner-link', 'owner-name', 'owner-avatar'];
 
+	// Never reorder or remove entries from this list. Just add new ones at the end, if needed.
+	// The item-activity table only stores the index and needs this array to know the matching activity.
 	const ACTIVITIES = [ACTIVITY_LIKE, ACTIVITY_DISLIKE, ACTIVITY_ATTEND, ACTIVITY_ATTENDNO, ACTIVITY_ATTENDMAYBE];
 
 	/**