Merge branch...

Merge branch '110-let-next-prev_lesson-return-none-if-the-person-isn-t-a-participant-of-the-lesson' into 'master'

Resolve "Let next/prev_lesson return None if the person isn't a participant of the lesson"

Closes #110

See merge request !132

aleksis/apps/chronos/model_extensions.py

@@ -97,6 +97,12 @@ def _adjacent_lesson(
         return None
 
     ids = list(daily_lessons.values_list("id", flat=True))
+
+    # Check if the lesson period is one of the person's lesson periods on this day
+    # and return None if it's not so
+    if lesson_period.pk not in ids:
+        return None
+
     index = ids.index(lesson_period.pk)
 
     if (offset > 0 and index + offset < len(ids)) or (offset < 0 and index >= -offset):