MDL-28006 restore - improved handling of gradebook 1.9 backups
On contrary from a backup created in 2.0, the file gradebook.xml in the
converted MBZ can contain the course grade_item without the
corresponding grade_category record which is something restore_stepslib
did not expect. This patch fixes two places:
1) the gradebook restore does not use mapped itemid but calls
grade_category::fetch_course_category() to get the actual category to
link the course_item with
2) after_execute makes sure that the mapping actually exists before
trying to move the activities from the default root category