Question

I found this wonderful NSManagedObjectID. This would be very good for referencing an Entity/NSManagedObject/NSEntityDescription, right?
Let’s get an ID from an entity:

NSEntityDescription *entity = [self newEntity];      NSManagedObjectID *objID = [entity objectID]; 

So… any idea how to get this objID into a string? Or better: NSData. Actually something to be able to save it to the NSUserDefaults. 😉

Btw: NSFetchRequest doesn’t want to work in my case. I use an modified version of this example: answer of an old question.

Answer 1

To get an archived URI corresponding to a NSManagedObject‘s objectID:

NSManagedObject* myMO; ... NSURL *uri = [[myMO objectID] URIRepresentation]; NSData *uriData = [NSKeyedArchiver archivedDataWithRootObject:uri]; 

In order to get back to an instance of the original managed object, you need a CoreData stack with the persistent store holding that instance already added to the NSPersistentStoreCoordinator. Then:

NSData *uriData; NSPersistentStoreCoordinator *psc; NSManagedObjectContext *moc; //with moc.persistentStoreCoordinator = psc. ... NSURL *uri = [NSKeyedUnarchiver unarchiveObjectWithData:uriData]; NSManagedObjectID *moID = [psc managedObjectIDForURIRepresentation:uri]; NSManagedObject *myMO = [moc objectWithID:moID];