'Bulk creates VisibleBlocks for the given iterator of
BlockRecordList objects for the given course_key, but
only for those that aren\'t already created.'

@classmethod
def bulk_get_or_create(cls, block_record_lists, course_key):

    existent_records = cls.bulk_read(course_key)
    non_existent_brls = {brl for brl in block_record_lists if (brl.hash_value not in existent_records)}
    cls.bulk_create(course_key, non_existent_brls)
