Permanent NSManagedObjectID not so permanent?

Question

I\'m having trouble dealing with object IDs in CoreData. I\'m using MagicalRecord for convenience and have 3 contexts: a private queue working context, a main queue context

Answer 1

Short answer is, don't do that :)

-objectID is not reliable between launches of your application. It is guaranteed to be unique and reliable under the following conditions:

1. Within a single lifecycle of the application
2. In its original object form (not in URL or NSString form)

Treating the -objectID as a permanent unique identifier to be stored outside of the persistent store is going to fail you fairly often. Core Data changes the underlying details of the -objectID many times during the life of the object.

If you need an externally reliable unique then you need to create one yourself. I generally recommend adding a [[NSProcessInfo processInfo] globallyUniqueString] to any entity that needs an externally reference-able unique. -awakeFromInsert is a great place to do that.

Answer 2

This may be because you're using nested context.

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

static NSArray *fetchAllPersons(NSManagedObjectContext *moc);
static NSManagedObjectModel *managedObjectModel();
static NSManagedObjectContext *createManagedObjectContext();
static NSURL *desktopDirectoryURL(void);

static void testObjectID(void);



int main(int argc, const char * argv[])
{
    @autoreleasepool {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            testObjectID();
        });
    }
    dispatch_main();
    return 0;
}




static void testObjectID(void)
{
    NSManagedObjectContext *c = createManagedObjectContext();
    [c performBlock:^{

        NSArray *all = fetchAllPersons(c);
        NSLog(@"count: %lu", all.count);
        NSLog(@"all accounts = %@", all);

        NSManagedObject *a = [NSEntityDescription insertNewObjectForEntityForName:@"Person" inManagedObjectContext:c];
        [a setValue:@"foo" forKey:@"name"];

        NSLog(@"temp a.objectID = %@", a.objectID);
        NSError *error = nil;
        NSCAssert([c obtainPermanentIDsForObjects:@[a] error:&error], @"perm id error: %@", error);
        NSLog(@"perm a.objectID = %@", a.objectID);

        NSCAssert([c save:&error], @"Save failed: %@", error);

        NSURL *u = a.objectID.URIRepresentation;

        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            NSManagedObjectContext *d = createManagedObjectContext();

            NSArray *all = fetchAllPersons(c);
            NSLog(@"count: %lu", all.count);
            NSLog(@"all accounts = %@", all);

            NSManagedObjectID *i = [d.persistentStoreCoordinator managedObjectIDForURIRepresentation:u];
            NSError *objWithIdError = nil;
            NSManagedObject *o = [d existingObjectWithID:i error:&objWithIdError];
            NSCAssert(o != nil, @"existing object error: %@", objWithIdError);

            NSLog(@"o = %@", o);
            NSLog(@"o.objectID = %@", o.objectID);
        });
    }];
}


static NSArray *fetchAllPersons(NSManagedObjectContext *moc)
{
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Person"];
    NSError *error = nil;
    NSArray *result = [moc executeFetchRequest:request error:&error];
    NSCAssert(result != nil, @"Fetch failed: %@", error);
    return result;
}

static NSManagedObjectModel *managedObjectModel()
{
    static NSManagedObjectModel *mom = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSEntityDescription *personEntity = [[NSEntityDescription alloc] init];
        [personEntity setName:@"Person"];

        NSAttributeDescription *nameAttribute = [[NSAttributeDescription alloc] init];

        [nameAttribute setName:@"name"];
        [nameAttribute setAttributeType:NSStringAttributeType];

        [personEntity setProperties:@[nameAttribute]];

        mom = [[NSManagedObjectModel alloc] init];
        [mom setEntities:@[personEntity]];
    });
    return mom;
}


static NSManagedObjectContext *createManagedObjectContext()
{
    static NSPersistentStoreCoordinator *coordinator;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel: managedObjectModel()];

        NSString *STORE_TYPE = NSSQLiteStoreType;
        NSString *STORE_FILENAME = @"foobar.db";

        NSError *error;
        NSURL *url = [desktopDirectoryURL() URLByAppendingPathComponent:STORE_FILENAME];

        NSPersistentStore *newStore = [coordinator addPersistentStoreWithType:STORE_TYPE
                                                                configuration:nil URL:url options:nil
                                                                        error:&error];

        if (newStore == nil) {
            NSLog(@"Store Configuration Failure\n%@",
                  ([error localizedDescription] != nil) ?
                  [error localizedDescription] : @"Unknown Error");
        }
    });

    NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    [moc setPersistentStoreCoordinator:coordinator];

    return moc;
}


static NSURL *desktopDirectoryURL(void)
{
    static NSURL *URL;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSFileManager *fileManager = [[NSFileManager alloc] init];
        NSError *error;
        URL = [fileManager URLForDirectory:NSDesktopDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:YES error:&error];
        NSCAssert(URL != nil, @"Could not access Desktop directory: %@", [error localizedDescription]);
    });
    return URL;
}

Outputs:

count: 0
all accounts = (
)
temp a.objectID = 0x10180e640 <x-coredata:///Person/tB1D48677-0152-4DA9-8573-7C7532863B4E2>
perm a.objectID = 0x101901bb0 <x-coredata://275C90E5-2598-4DFA-BF4C-E60A336E8BE4/Person/p1>
count: 1
all accounts = (
    "<NSManagedObject: 0x10180e5b0> (entity: Person; id: 0x101901bb0 <x-coredata://275C90E5-2598-4DFA-BF4C-E60A336E8BE4/Person/p1> ; data: {\n    name = foo;\n})"
)
o = <NSManagedObject: 0x100416530> (entity: Person; id: 0x100415b60 <x-coredata://275C90E5-2598-4DFA-BF4C-E60A336E8BE4/Person/p1> ; data: {
    name = foo;
})
o.objectID = 0x100415b60 <x-coredata://275C90E5-2598-4DFA-BF4C-E60A336E8BE4/Person/p1>