Need to check whether an item exists before updating

[vcardins]

Hi, I need to be able to check whether the item that I want to update exists so I can write the correct predicate.
How can I add a vote item if I don't know whether it already exists?

items[id=67fa931b-d0e3-48c0-80ef-49f34559117e].notes[id=75ad2485-56c0-4878-99c4-ef621dcb0445].votes[userId=b6fdb697-1e92-46ae-83cc-ecb724a66f6e]