Fix #813 - delete grades before main record (MDL-64878)

Author: danmarsden

Diff for: lib.php

@@ -166,10 +166,11 @@ function attendance_delete_instance($id) {
 
     $DB->delete_records('attendance_warning', array('idnumber' => $id));
 
-    $DB->delete_records('attendance', array('id' => $id));
-
+    // Grades must be deleted before the main attendance record.
     attendance_grade_item_delete($attendance);
 
+    $DB->delete_records('attendance', ['id' => $id]);
+
     return true;
 }