Core Data using auto generated Primary Key PK

Question

I have a simple Person entity in Core Data, I noticed that when adding data, each row is auto generated a unique PK called objectID

I wish to retrieve the records using the objectID but I am getting the following error:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'keypath objectID not found in entity < NSSQLEntity Person id=2 >'

func fetchPerson(withID personID: Int,
                 context: NSManagedObjectContext,
                 completion: @escaping ([Person]?) -> Void) {

    let fetchRequest: NSFetchRequest<Person> = Person.fetchRequest()

    let predicate = NSPredicate(format: "\(#keyPath(Person.objectID)) == \(personID)")
    fetchRequest.predicate = predicate

    //perform aynchronous operation:
    context.perform {
        do {
            let persons = try fetchRequest.execute()
            //success - return array of persons
            completion(persons)
        } catch {
            //error - return nil
            completion(nil)
        }

    }

}

Calling the above :

//fetch person with specific id:
let personID: Int = 2
fetchPerson(withID: personID, context: persistentContainer.viewContext) { (persons: [Person]?) in
    if persons != nil {

        print("Fetching person with personID: \(personID)")
        for person in persons! {
            print("Person: \(person.objectID) - \(person.firstName!) \(person.lastName!)")
        }

    }
}

Jon Rose Jon Rose · Accepted Answer · 2017-03-29T06:36:26

You can fetch with objectID by using the predicate "self == %@", objectID. An objectID is property of a managedObject and its type is NSManagedObjectID not an Int. In general it is better to create and manage your own objectId. I generally use UUID strings.

Core Data using auto generated Primary Key PK

2 Answers