Including occupied username and renaming DB field

Now it shows the username of who is using an item.
Item db field 'usedBy' was renamed to 'used_by' to keep consistence.

app/Item.php

@@ -23,7 +23,7 @@ class Item extends Model
         return $query->where([
             ['user_id', $user_id],
             ['product_id', $product_id],
-            ['usedBy', ''],
+            ['used_by', ''],
         ]);
     }