Currently it seems like there is no way to return a subset of items in a collection. If I wanted to find items in a collection that matched some type of 'filter', I would have to retrieve all the items in the collection and filter myself to get this subset. Here is API docs:
For example, lets say I have a site that lists Albums by artists. So I have a 2 collections:
One of the fields in the Album collection is 'artist_id'. Lets say my favorite artist is Taylor Swift, which it is, there is no way for me to search the Album collection for: artist_id = taylor_swift_id.
I would think this is a functionality that many developers would love to see.