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 field. I'm not seeing anything like that here, but I could just be missing something. I just want an auto incrementing NSInteger field, so when I manually add items to the database, I will have some form of reference to them.

Solution 1

Though i agree with what you said, there is an ID mechanism behind core data. ID are indeed managed by Core-Data but you can retrieve them with :

NSManagedObjectID *moID = [managedObject objectID];

For more informations see : Core Data Programming Guide

Solution 2

That ain't how CoreData works.

In CoreData, you create instances of entities. Each instance is unique. You then retrieve and manipulate instances as needed. CoreData takes care of the persistence for you, including uniquely identifying instances.

Step away from everything you know about traditional relational databases.

CoreData is an awesomely powerful bit of technology that offers features well beyond just database like persistence. It'll save you many lines of code and perform extremely well if you embrace it.

Solution 3

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<NSFetchRequestResult>.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

}

Solution 4

Take a look at NSProcessInfo / ProcessInfo from where we can fetch unique id.

Objective-C code snippet:

[[NSProcessInfo processInfo] globallyUniqueString].

Swift code snippet:

let uniqueId = ProcessInfo().globallyUniqueString
print("uniqueId: \(uniqueId)")

Apple Docs:

Global ID for the process. The ID includes the host name, process ID, and a time stamp, which ensures that the ID is unique for the network, which is handy if you don't want people guessing the id.

Solution 5

Here are three kinds of unique ID representation of CoreData object:

NSManagedObjectID *taskID = [task objectID];
NSURL *taskUniqueURLKey = task.objectID.URIRepresentation;
NSString *taskUniqueStringKey = task.objectID.URIRepresentation.absoluteString;

!!!!Note: if you want to use above unique key as index, please make sure the object is saved to context before using it , or the objectID would be a temporary Id, which will be replaced later (e.g. after saved). See the apple document of NSManagedObjectID class :

- (BOOL)isTemporaryID;    // indicates whether or not this ID will be replaced later, 
such as after a save operation (temporary IDs are assigned to newly inserted objects 
and replaced with permanent IDs when an object is written to a persistent store); most
 IDs return NO

View more solutions

Comments

[Abhinav Kaushal Keshari]

In this case I would use two ids and store the server auto incrementing id as an attribute of the core data model.

[RonLugge]

It's regretful that the questioner never accepted such a wonderful answer.

[Ryan Poolos]

This was exactly what I needed over 2 years later. Also using the URIRepresentation of an NSManagedObjectID you can easily store these off in NSUserDefaults as was necessary for me in a caching scheme. ^1

[andrew k]

I disagree with the "step away from everything you know". There are cases where you might be given unique entities with no distinguishing properties (remove server that doesn't abide by rules). Having an auto-increment would be useful here.

[bbum]

Did you read the last sentence in OP's question ("I will have some form of reference to them.")? You don't need an automatically incrementing field for that. Per entity unique identifiers are already built into Core Data. There are certainly reasons why you might want an auto-incrementing field, but doing so for uniquely identifying entities is just re-inventing the wheel. Certainly, you might do it for correlation from a client to a server, but only if the server can't take the URI already supported by NSManagedObjectID.

[bbum]

Use whatever the server side uses to uniquely identify records; adding an auto-incrementing number on top will certainly work (Lukasz provides a very inefficient solution), but it is just another detail to break in the future.

[Manann Sseth]

Superb solution mate. Cheers !!

[Jason Moore]

To make this efficient, sort by descending index, and then set the fetch limit to 1. i.e. NSSortDescriptor *indexSort = [[NSSortDescriptor alloc] initWithKey:idKey ascending:NO]; and then uncomment request.fetchLimit = 1;

[bbum]

@DanBeaulieu Sigh. I really wish Apple would not break links all the time. I'll try to find the time to fix 'em.

[Mateus]

Be aware that the objectID can and does change, especially when a migration occurs. Do not rely on that value to be consistent between one launch of the application and the next. - Comment by Marcus S Zarra in another thread.

[John]

Almost Perfect! Even better with @JasonMoore 's optimization!

[vomi]

Do not use objectID. First, it can be temporary and change when the NSManagedObject is saved. Second it might not be consistent between app launches.

[Justin]

Object IDs might also change during the database migrations or after changing the name of a table.

[Jayprakash Dubey]

What will be equivalent in Swift?