'Returns the latest credit request of user against the given course.
Args:
username(str): The username of requesting user
course_key(CourseKey): The course identifier
Returns:
CreditRequest if any otherwise None'

@classmethod
def get_user_request_status(cls, username, course_key):

    try:
        return cls.objects.filter(username=username, course__course_key=course_key).select_related('course', 'provider').latest()
    except cls.DoesNotExist:
        return None
