Add model extension to Person for check if the person is a teacher

aleksis/apps/chronos/model_extensions.py

@@ -3,6 +3,13 @@ from aleksis.core.models import Person
 from .models import Lesson, LessonPeriod
 
 
+@Person.property
+def is_teacher(self):
+    """ Check if the user has lessons as an teacher """
+
+    return Lesson.objects.filter(teachers=self).exists()
+
+
 @Person.property
 def lessons_as_participant(self):
     """ Return a `QuerySet` containing all `Lesson`s this person