I would consider this model "usually incorrect" in that it is quite likely you will eventually want to be able to move an entity to a different parent. I would thereby always give people the advice that one "should" have primary keys for every entity type that are at least global for that local entity type (if not truly global for all entity types, as there is a lot of power doing such) and then implement this child relationship as a unique foreign key constraint, unless you have some very specific functionality or performance requirement that would force primary key conflation (and FWIW, "I want to use this WAL watcher" might count, but it seems like a steep price to pay being unable to do sane reparenting).