接口成员是抽象的吗？(Are interface members abstract?)

 你们的关系应该都是反向的（在这种情况下，彼此相反）。 这允许Core Data正确管理关系，并允许您在两个方向导航时方便地设置它。 它也恰当地代表了这种关系。 
 
 谓词应该是： 
 
NSPredicate *p = [NSPredicate predicateWithFormat:@"camera == %@", self.selectedCamera];
 
 在Roll实体上使用camera关系并检查它是否包含提供的self.selectedCamera实体实例。 

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.