Queryset for all distinct authors this course had so far. Important for statistics.
            Note that this may be different from the list of people being registered for the course,
            f.e. when they submit something and the leave the course.

def authors(self):
        '''
            Queryset for all distinct authors this course had so far. Important for statistics.
            Note that this may be different from the list of people being registered for the course,
            f.e. when they submit something and the leave the course.
        '''
        qs = self._valid_submissions().values_list('authors',flat=True).distinct()
        return qs