Core Data: Save unique object ID

I see that there is a method of getting the unique id of a managed object:

NSManagedObjectID *moID = [managedObject objectID];

But, as I have read, that changes when it is saved.

What is a better way of creating my own unique ID and saving it in each object, making sure tha开发者_运维知识库t it IS unique and does't change?

You cannot save the NSManagedObjectID in a CoreData entity, and there's no properties in it that can be intended as integer or string ID. Therefore building your own unique identifier algorithm is an acceptable solution, if it's impossible for you to keep track when the entity is saved, I did this for some applications.

For example I had a similiar problem time ago, when I needed to fill a UITableView cells with a reference to the entity, and retrieve the entity after clicking/touching the cell.

I found a suggestion by using the uri representation, but however I still needed to save the context before, but I manage to use the NSFetchedResultsController and found a more solid solution rather than an application built id.

[[myManagedObject objectID] URIRepresentation];

Then you can later retrieve the managed object id itself:

NSManagedObjectID* moid = [storeCoordinator managedObjectIDForURIRepresentation:[[myManagedObject objectID] URIRepresentation]];

And with the moid I could retrieve your managed object.

I have a created date property in my object, so I just ended up using that date including seconds, which, seems to me like it will be unique and work how I want.

You can create an id field for your object, and populate it during init using GUID. for how to create a GUID and optionally export it to string see UUID (GUID) Support in Cocoa

If it helps anyone else searching for this, this is how I do it:

Create an ID property on the object

Get the last used ID when creating an object with this code:

Playlist *latest;

// Define entity to use and set up fetch request
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Playlist" inManagedObjectContext:managedObjectContext];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:entity];


// Define sorting
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"listID" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];


// Fetch records and handle errors
NSError *error;
NSArray *fetchResults = [managedObjectContext executeFetchRequest:request error:&error];

if (!fetchResults) {
    NSLog(@"ERROR IN FETCHING");
}

if ([fetchResults count] == 0) {
    latestID = 0;
} else {
    latest = [fetchResults objectAtIndex:0];
    latestID = latest.listID;
}

Increment this by one on the new object.