Improve fix for #1516 about deleting courses with skills assigned

main/inc/lib/course.lib.php

@@ -2333,7 +2333,9 @@ class CourseManager
 
             // Skills
             $table = Database::get_main_table(TABLE_MAIN_SKILL_REL_USER);
-            $sql = "DELETE FROM $table WHERE course_id = $courseId";
+            $argumentation = Database::escape_string(sprintf(get_lang('SkillFromCourseXDeletedSinceThen'), $course['code']));
+            error_log($argumentation);
+            $sql = "UPDATE $table SET course_id = NULL, session_id = NULL, argumentation = '$argumentation' WHERE course_id = $courseId";
             Database::query($sql);
 
             // Delete the course from the database