I am not sure i understand what you are saying about guarantees. If there is a FK, there is a relation. It's the user that is driving the relations by specifyi

I am not sure i understand what you are saying about guarantees. If there is a FK, there is a relation. It's the user that is driving the relations by specifying what columns are FK, it's the same thing as defining relations in a GUI, only you do it at the database level