Returns the list of courses this user is subscribed for,
            or owning, or tutoring.
            This leads to the fact that tutors and owners don't need
            course membership.

def user_courses(self):
        '''
            Returns the list of courses this user is subscribed for,
            or owning, or tutoring.
            This leads to the fact that tutors and owners don't need
            course membership.
        '''
        registered = self.courses.filter(active__exact=True).distinct()
        return (self.tutor_courses() | registered).distinct()