Provide unified coursebook for similar lessons

aleksis/apps/alsijil/schema.py

@@ -234,7 +234,7 @@ class Query(graphene.ObjectType):
 
         return LessonDocumentation.objects.filter(
             lesson_period_id__in=LessonPeriod.objects.filter(
-                lesson__in=lesson.equal_lessons
+                lesson__in=lesson._equal_lessons
             ).values_list("id", flat=True)
         ).order_by(
             "-year", "-week", "-lesson_period__period__weekday", "-lesson_period__period__period"