'Check whether the course has been configured for credit.
Args:
course_key (CourseKey): Identifier of the course.
Returns:
bool: True iff this is a credit course.'

@classmethod
def is_credit_course(cls, course_key):

    credit_courses = cache.get(cls.CREDIT_COURSES_CACHE_KEY)
    if (credit_courses is None):
        credit_courses = set((unicode(course.course_key) for course in cls.objects.filter(enabled=True)))
        cache.set(cls.CREDIT_COURSES_CACHE_KEY, credit_courses)
    return (unicode(course_key) in credit_courses)
