Broken caching in the grade item classes leads ot O(n) DB queries in the grader report

Index: lib/grade/grade_item.php

@@ -871,7 +871,7 @@

      * @return object Grade_category

      */

     function load_item_category() {

-        if (empty($this->category->id)) {

+        if (empty($this->item_category->id)) {

             $this->item_category = $this->get_item_category();

         }

         return $this->item_category;