Core Data: Keypath Error Not Found in Entity <NSSQLEntity Studies id=3>

Either the Studies entity does not have a patient property, or whatever entity the patients relationship points to does not have a PatientName property (pay attention to upper/lowercase issues) or both.

Usually when I see this error it means that I added a new Version to my Model but forgot to update the Current Model Version for the project.

Posting this answer to help my future self because it's not the first time I have landed on this page!

Firstly, since your fetch entity is Studies you don't include it in the predicate because the Studies objects are the ones receiving the predicate test in the first place. So your predicate should be at least just:

patients.PatientName == %@

However, by convention, patients would indicate a to-many relationship. If so, that means that the actual value of patients is a set of (presumably) Patient objects. As such you can't ask a set for an attribute value as above: Instead you have to ask for a new set of all object in the set that match the predicate. Use the ANY or All operator like so:

ALL patients.PatientName == %@

I would add that by convention all attribute and relationship names start with lower case letters so if PatientName is an attribute it should be patientName.