Create private collection that tracks all ids of parent

[kentmw]

In some cases, a cache is treated less like a union-based collection for all private collections and instead like a collection of all elements from the server.
In this case, if I just want to own, sort, or manipulate in some way the cache itself without affecting other areas of the code, I'd want to create a private collection that always has the same elements as the cache itself. I believe right now I have to manage a private collection manually to track all the current ids of the cache.