I recall seeing one of the founders say in a post years back that UniqueID was handled differently by Bubble, which is why you can’t do an “is in” type of search. But regardless it’s a major pain. As a workaround you need to search a record based on some other field and ensure that field is unique. This may mean creating a text field and copying over the UniqueID into it, which feels messy but I don’t see a way around it.
In my case I’m running into this when trying to do a nested search of a joining table (which I’m building to try to avoid an advanced filter on a many-to-many search). If I have Items, and Tags, and an Item_Tag joining table, in my Item search constraints what I want to say in essence is… this Item is in search for Item_Tag’s Item. However there’s no such expression. Instead you have to pick a field and say… Item’s Field is in search for Item_Tag’s Item’s Field.