feat: update the index of orphan exercises

When an exercise is removed from a lesson, the link to the lesson is
removed from that exercise and the index is reset. This will make sure
the removed exercises won't show up in places like progress.

community/lms/doctype/lesson/lesson.py

@@ -16,6 +16,21 @@ class Lesson(Document):
                 e = s.get_exercise()
                 e.lesson = self.name
                 e.save()
+        self.update_orphan_exercises()
+
+    def update_orphan_exercises(self):
+        """Updates the exercises that were previously part of this lesson,
+        but not any more.
+        """
+        linked_exercises = {row['name'] for row in frappe.get_all('Exercise', {"lesson": self.name})}
+        active_exercises = {s.id for s in self.get("sections") if s.type=="exercise"}
+        orphan_exercises = linked_exercises - active_exercises
+        for name in orphan_exercises:
+            ex = frappe.get_doc("Exercise", name)
+            ex.lesson = None
+            ex.index_ = 0
+            ex.index_label = ""
+            ex.save()
 
     def get_sections(self):
         return sorted(self.get('sections'), key=lambda s: s.index)