Creating a unique id for a Core Data program on the iPhone

Question

I am having a bit of trouble figuring out this Core Data stuff. How do I create a new entry with a unique ID? In SQL I would just declare one field as an autoincrement fie

Answer 1

This class function will return next available number for your id property (it must be integer property).

I call it auto-increment value generator. I still agree with others that there is objectID for that but sometimes you just need number.

You can put this function in your NSManagedObject subclass for the entity:

+(NSNumber *)nextAvailble:(NSString *)idKey forEntityName:(NSString *)entityName inContext:(NSManagedObjectContext *)context{


    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSManagedObjectContext *moc = context;
    NSEntityDescription *entity = [NSEntityDescription entityForName:entityName inManagedObjectContext:moc];

    [request setEntity:entity];
    // [request setFetchLimit:1];

    NSArray *propertiesArray = [[NSArray alloc] initWithObjects:idKey, nil];
    [request setPropertiesToFetch:propertiesArray];
    [propertiesArray release], propertiesArray = nil;

    NSSortDescriptor *indexSort = [[NSSortDescriptor alloc] initWithKey:idKey ascending:YES];
    NSArray *array = [[NSArray alloc] initWithObjects:indexSort, nil];
    [request setSortDescriptors:array];
    [array release], array = nil;
    [indexSort release], indexSort = nil;

    NSError *error = nil;
    NSArray *results = [moc executeFetchRequest:request error:&error];
    // NSSLog(@"Autoincrement fetch results: %@", results);
    NSManagedObject *maxIndexedObject = [results lastObject];
    [request release], request = nil;
    if (error) {
        NSLog(@"Error fetching index: %@\n%@", [error localizedDescription], [error userInfo]);
    }
    //NSAssert3(error == nil, @"Error fetching index: %@\n%@", [error localizedDescription], [error userInfo]);

    NSInteger myIndex = 1;
    if (maxIndexedObject) {
        myIndex = [[maxIndexedObject valueForKey:idKey] integerValue] + 1;
    }

    return [NSNumber numberWithInteger:myIndex];
}

Swift 5.0

func nextAvailble(_ idKey: String, forEntityName entityName: String, in context: NSManagedObjectContext) -> NSNumber? {

    let req = NSFetchRequest.init(entityName: entityName)
    let entity = NSEntityDescription.entity(forEntityName: entityName, in: context)
    req.entity = entity
    req.fetchLimit = 1
    req.propertiesToFetch = [idKey]
    let indexSort = NSSortDescriptor.init(key: idKey, ascending: false)
    req.sortDescriptors = [indexSort]

    do {
        let fetchedData = try context.fetch(req)
        let firstObject = fetchedData.first as! NSManagedObject
        if let foundValue = firstObject.value(forKey: idKey) as? NSNumber {
            return NSNumber.init(value: foundValue.intValue + 1)
        }

        } catch {
            print(error)

        }
    return nil

}