Your relationships should each have an inverse (in this case, be inverse of each other). This allows Core Data to properly manage the relationship and allows you to set it conveniently while navigating it in both directions. It also properly represents the relationship.
The predicate should be:
NSPredicate *p = [NSPredicate predicateWithFormat:@"camera == %@", self.selectedCamera];
to use the camera
relationship on the Roll
entity and check that it contains the provided self.selectedCamera
entity instance.