This was a one-time use application but I wanted to store it for future reference.
I had originally nested data such that:
cats (collection) --> cats (documents) --> LitterTrips (collection) --> LitterTrips. where cats had cat details like ID, Name, age, color, photoURL and where LitterTrips had details like time of trip, probability of trip, direction
litterTrips (collection) --> litterTrips (documents) where litterTrips have trip detail and CatID and CatName
cats (collection) still houses more detail about cats catid, catname, color, age, photoURL
The focus of the app(s) using the data is trips to the litterbox, not cats. The user wants to see the lates trips to the box regardless of cat.
We can still look for trips from a particular cat, but swapping the focus made the app much simpler.