code: def get_course_user_aggregation(self, course, username=None):
          
          if username is None:
              username = self.session_username()
  
          return self._database.aggregations.find_one({"courseid": course.get_id(), "students": username})
docs: Returns the classroom whose username belongs to
          :param course: a Course object
          :param username: The username of the user that we want to register. If None, uses self.session_username()
          :return: the classroom description