Codeigniter cache

$cacheID = 'assignments_calendar_'.$id;
if (!$assignments = $ci->cache->get($cacheID)){
    $assignments = $ci->model->get_calendar_assignments($id);
    $ci->cache->save($cacheID, $assignments, 60*60*24);
}

if ($this->cache->get('assignments_calendar_'.$id)) {
    $this->cache->delete('assignments_calendar_'.$id);
}