Merge branch '118-check-if-getters-for-lesson-periods-work-correctly' into 'master'

Resolve "Check if getters for lesson periods work correctly"

Closes #118

See merge request !141

aleksis/apps/chronos/models.py

@@ -453,14 +453,16 @@ class LessonPeriod(WeekAnnotationMixin, TeacherPropertiesMixin, ExtensibleModel)
         return None
 
     def get_subject(self) -> Optional[Subject]:
-        if self.get_substitution() and self.get_substitution().subject:
-            return self.get_substitution().subject
+        sub = self.get_substitution()
+        if sub and sub.subject:
+            return sub.subject
         else:
             return self.lesson.subject
 
     def get_teachers(self) -> models.query.QuerySet:
-        if self.get_substitution():
-            return self.get_substitution().teachers
+        sub = self.get_substitution()
+        if sub and sub.teachers.all():
+            return sub.teachers
         else:
             return self.lesson.teachers