'Return a dict mapping course_ids to CourseOverviews, if they exist.
This method will *not* generate new CourseOverviews or delete outdated
ones. It exists only as a small optimization used when CourseOverviews
are known to exist, for common situations like the student dashboard.
Callers should assume that this list is incomplete and fall back to
get_from_id if they need to guarantee CourseOverview generation.'

@classmethod
def get_from_ids_if_exists(cls, course_ids):

    return {overview.id: overview for overview in cls.objects.select_related('image_set').filter(id__in=course_ids, version__gte=cls.VERSION)}
