function deleteCachedItemsContainingSongId(Song $song)
{
// get all items that refer this song
$items = $song->items()->get();
$item_id_list = [];
// get each item id into an array
foreach ($items as $item) {
# find item in cache
array_push($item_id_list, $item->id);
}
// get all items in the cache with the same item_id's
$cachedItems = PlanCache::whereIn('item_id', $item_id_list);
$cachedItems->delete();
}