Resolve "Handle UniqueConstraint errors while saving coursebook"

Closes #338 (closed)

aleksis/apps/alsijil/models.py

@@ -302,14 +302,13 @@ class Documentation(CalendarEvent):
                 lesson_event.teachers,
             )
 
-        obj, __ = cls.objects.get_or_create(
+        obj, __ = cls.objects.update_or_create(
             datetime_start=datetime_start,
             datetime_end=datetime_end,
-            course=course,
-            defaults=dict(subject=subject, amends=lesson_event),
+            amends=lesson_event,
+            defaults=dict(subject=subject, course=course),
         )
         obj.teachers.set(teachers.all())
-        obj.save()
 
         # Create Participation Statuses
         obj.touch()