From http://stackoverflow.com/questions/2239797/core-data-unique-attributes Unique Item in Core Data

coredata_uniqueitem

+ (TZUser *)userWithUniqueUserId:(NSString *)uniqueUserId inManagedObjectContext:(NSManagedObjectContext *)context
{
    TZUser *user = nil;
    NSFetchRequest *request = [[NSFetchRequest alloc] init];

    request.entity = [NSEntityDescription entityForName:@"TZUser" inManagedObjectContext:context];
    request.predicate = [NSPredicate predicateWithFormat:@"objectId = %@", uniqueUserId];
    NSError *executeFetchError = nil;
    user = [[context executeFetchRequest:request error:&executeFetchError] lastObject];

    if (executeFetchError) {
         NSLog(@"[%@, %@] error looking up user with id: %i with error: %@", NSStringFromClass([self class]), NSStringFromSelector(_cmd), [uniqueUserId intValue], [executeFetchError localizedDescription]);
    } else if (!user) {
        user = [NSEntityDescription insertNewObjectForEntityForName:@"TZUser" 
                                             inManagedObjectContext:context];
    }

    return user;
}