final class Author: NSManagedObject {
    @NSManaged var name: String?
    @NSManaged public var publications: NSSet?
}

class Publication: NSManagedObject {
    @NSManaged var publicationDate: Date?
    @NSManaged var numberOfViews: Int64
    @NSManaged var author: Author?
}

final class Article: Publication {
    @NSManaged var text: String?
}