'Retrieve all course keys for a particular org.
Arguments:
org_aliases (list): List of aliases for the org.
Returns:
List of `CourseKey`s'

def _get_courses_for_org(self, org_aliases):

    all_courses = modulestore().get_courses()
    orgs_lowercase = [org.lower() for org in org_aliases]
    return [course.id for course in all_courses if (course.id.org.lower() in orgs_lowercase)]
