Save a NSManagedObjectID in a plist file

Question

I'm developing an app that uses CoreData. For the purpose of my app, I would like to save the managedObjectId of an object (in a plist file) in order to retrieve this object on the next app launch.

Following some reseaches, I tried :

myDictionary setObject:self.myPerson.objectID.URIRepresentation.absoluteString forKey:@"lastSelectedPerson"];
// Then I write the dictonary into a file
// I get a value like "x-coredata://8CF33004-BADD-402D-9AA5-115A030F901A/Person/p1"

myPerson is a object that has been save in the managedObjectContext.

Next I have tried to retrieve the objectID, but I get a nil value with this code :

// After retrieving the dictonary from the plist file
NSString * lastSelectedPersonId = [myDictionary objectForKey:@"lastSelectedPerson"];
NSManagedObjectID * objectID = [self.managedObjectContext.persistentStoreCoordinator managedObjectIDForURIRepresentation:[NSURL URLWithString:lastSelectedPersonId]];
self.myPerson = (Person *)[self.managedObjectContext existingObjectWithID:objectID error:&error];

Do you have any suggestion ? Thanks.

Answer 1

To restore object on next launch you can alternatively try following:

1. Save objectID to NSUserDefaults:

   [[NSUserDefaults standardUserDefaults] setObject:self.myPerson.objectID forKey:@"lastSelectedPerson"]; 
   [[NSUserDefaults standardUserDefaults] synchronize];
   

2. On the next launch get value from NSUserDefaults:

   NSNumber *lastSelectedPersonID = [[NSUserDefaults standardUserDefaults] objectForKey:@"lastSelectedPerson"];
   

3. Query Core Data store to get object:

   - (Person *)personWithID:(NSNumber *)personID {
   
   Person *person = nil;
   
   NSFetchRequest *request = [[NSFetchRequest alloc] init];
   request.entity = [NSEntityDescription entityForName:@"Person" inManagedObjectContext:self.managedObjectContext];
   request.predicate = [NSPredicate predicateWithFormat:@"objectID == %@", personID];
   
   NSError *error = nil;
   NSArray *persons = [self.managedObjectContext executeFetchRequest:request error:&error];
   
   person = [persons lastObject];
   
   return person;
   }
   

Answer 2

I had a similar problem that I was creating a managed object and pulling off the object id and it hadn't been saved to disk yet. If that is the case the object id is a "temporary" id. I would check using: [NSManagedObjectID isTemporaryID]

If it returns that it is a temporary id you need to save the object and make sure you have a permanent id.

Documentation: http://developer.apple.com/library/mac/documentation/Cocoa/Reference/CoreDataFramework/Classes/NSManagedObjectID_Class/Reference/NSManagedObjectID.html#//apple_ref/occ/instm/NSManagedObjectID/isTemporaryID