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

def get_course_user_aggregation(self, course, username=None):
        """ 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
        """
        if username is None:
            username = self.session_username()

        return self._database.aggregations.find_one({"courseid": course.get_id(), "students": username})