'Reads a grade from database
Arguments:
user_id: The user associated with the desired grade
usage_key: The location of the subsection associated with the desired grade
Raises PersistentSubsectionGrade.DoesNotExist if applicable'

@classmethod
def read_grade(cls, user_id, usage_key):

    return cls.objects.select_related('visible_blocks').get(user_id=user_id, course_id=usage_key.course_key, usage_key=usage_key)
