Allow filtering based on size of arrays

[shaunbyrnes]

Hey guys,

We have the situation where some of the TS collections synced from the source DB Firestore have sub collections. The sub collections of course don't come across, but are sometimes needed to be considered in the queries that we perform.

Currently we have some arrays in the root level collection in Firebase/Firestore that store IDs for docs in the sub collections so they can be referenced, such as items[]. These arrays are at least already being written to, so if we could filter on the array length or items in the array with something like filter_by: count(items) >= 0 or the like, would be great.