global identifiers? - iCloud + Core Data + Ensembles - duplicates when deleting objects

Question

I am trying to implement iCloud sync in my Core Data app. I am not that pro in programming and this is really an advanced topic I learned... I found that Core Data sync Framework "Ensembles" by Drew McCormack. It seems to make iCloud Sync much easier.

I integrated it in my App and syncing does work quite well as long as I add new objects to my Core Data model. But when I delete an object, it creates duplicates. And then duplicates from duplicates. I ended up having the same Entry (object) like 3-4 times...

Why is that? What am I doing wrong? I did some research and my guess is that global identifiers could solve this?

What are global identifiers? My guess is that they help to avoid duplicates!? But how do I set this? I really have no idea, did a lot of research but couldn´t find an answer to that.

Thanks for help!

Update: Thanks for help! I read the readme and the book, but since i am beginner not everything is clear to me.

I think I understand the use of global identifiers in Ensembles now, but I don´t know if I´m doing it correctly.

If I understand it right, I have to assign an identifier to each object. I can do this by storing it in an attribute. This identifier can be anything as long as it is unique and a NSString?

In my app the user can store different things, let´s say name, text, title, date and so on. The app is based on the Master-Detail-View template in Xcode and uses Core Data. My Core Data model has only a single entity with some attributes, most are strings and a NSDate. No relationships or anything. If the user hits "+" a new object is created and I store the things the user enters in the attributes.

What I did to add global identifiers is to add a new attribute that stores it. So when a new object is created i do

/// I did find that to use as identifier !?

NSString *taskUniqueStringKey = newManagedObject.objectID.URIRepresentation.absoluteString;

/// and store it in the attribute.

[newManagedObject setValue:taskUniqueStringKey forKey:@"coreDataObjectID"]; 

Then i use this:

- (NSArray *)persistentStoreEnsemble:(CDEPersistentStoreEnsemble *)ensemble globalIdentifiersForManagedObjects:(NSArray *)objects
{

return [objects valueForKeyPath:@"coreDataObjectID"];;

}

This seems to work for me. But am I doing it right? Is this the right place to assign a global identifier? I have no awakeFromInsert !?

If this is working, I got the next problem. My app is already live and older entries that the user saved before the update will be missing the global identifier. What can I do about that? I thought what I already got and what is unique and the only thing I can think of is an attribute that saves [NSDate date] when the object is created.

I was trying to use this but I failed because Ensembles will only accept NSString and not NSDate!? Can I use this date attribute, is this unique enough and working as gloabl identifier? And if yes, could you please give me code example in how to convert this from date to string?

Syncing with Ensembles works quite good. No duplicates anymore, you can just switch off iCloud and the entries stay and switch it on again and it syncs like it should without loosing locally stored objects or so. Ensembles is really cool! I am seeing some minor strange behaviors like sometimes sync takes long, sometimes it´s really quick and if I edit things in a short time period on two different devices it gets a bit messed up like an object that I just deleted reappears. But I guess that´s normal? If I take some time between using the app on the different devices everything works fine.

Do I understand it right, there is only that one method to call for sync:

- (void)syncWithCompletion:(void(^)(void))completion
{
if (self.ensemble.isMerging) return;


if (!self.ensemble.isLeeched) {
    [self.ensemble leechPersistentStoreWithCompletion:^(NSError *error) {
        if (error) NSLog(@"Error in leech: %@", error);

        if (completion) completion();
    }];
}
else {
    [self.ensemble mergeWithCompletion:^(NSError *error) {

        if (completion) completion();
    }];
}

and you just call it if needed? There is nothing else like doing merge without leeching before, or a method like "this is the actual status - save it like it is now" ?

There are different points in the app where you want to sync. On app start and when terminating will be a good point. In my app there are two points where I should sync I guess: when adding an object and save it to Core Data and when I save changes to the object. I could also provide a button like "sync now". Is this a good approach and do I always just call

[self syncWithCompletion:NULL];

Another question that came up. Can I exclude objects from sync with Ensembles? My app loads tutorial entries as objects once on first app start. I don´t want to sync them if that´s possible somehow?

Thanks a lot for your help! If I could help you with anything like localizing in german or so let me know ! ;)

Answer 1

Yes, this is almost certainly due to not setting up global identifiers for your objects, or at least not doing it properly.

When you leech your ensemble, the local persistent store is imported into the sync data. Without global identifiers, Ensembles will assign random ids to your objects, so it can track them across devices.

Duplicates arise when you leech a second device that has the same data. Ensembles has no way to know that the data represents the same logical objects as on the other device, so it again assigns random ids. Effectively, it treats the objects on each device as being completely independent, so that all end up in your data set after syncing.

The solution is global identifiers. By implementing a CDEPersistentStoreEnsemble delegate method, you can provide Ensembles with global ids, which it can use to identify which objects on different devices belong together.

What should you use for global ids? Often, just a UUID, though for singleton like objects you will just want to pick an id.

You can initialize them in awakeFromInsert. You can store the global ids in attributes on your entities. (Note that if you are migrating an existing app, you will want to check with a fetch if the global ids have been generated BEFORE you try to leech the store for syncing.)

More details are in the README on GitHub and in the book at leanpub.

Update

To answer your update questions:

Yes, an identifier just has to be a string, and immutable. It should not change once assigned.

The NSManagedObjectID is not a very good global identifier, in that it will be different on different devices. We really want something that is global across devices.

If you are starting from scratch, using NSUUID is a good approach. Just create a unique id, and store it in the object.

If you have an existing app, and it has been syncing via another mechanism, you need to come up with a way to provide the same global identifiers on each device. One way to do that is mash up the object properties in some way. Usually that will give you a pretty-close-to-unique value, and it will be good enough for the transition.

As an example, you do a quick fetch, and discover that your objects don't yet have global ids. You go through the objects, and set the global ids to a string comprised of creationDate + text. (You could even shorten this by taking a hash, but it probably isn't that important.) After this initial 'migration' to global identifiers, you would just use UUIDs for any newly created objects.

Note that you don't have to use awakeFromInsert. That is simply a convenient place to put it. As long as you assign the global identifier before saving the object you should be fine.

The easiest way to get a string from an NSDate is to call the description method, but another way would be to get a double using timeIntervalSince1970, and turning that into a string. (Be careful with dates as unique identifiers on their own: often objects created together will have the same creation date.)

You are correct about how you should do a sync: you can simply call syncWithCompletion:.

To answer the question about excluding objects: You can't exclude individual objects, mainly because it could become tricky when those objects have relationships to synced objects. You can handle these objects in one of two ways:

1. Put them in a separate persistent store, and add that store to the same persistent store coordinator.
2. Sync the objects, but give them global ids manually, so that the objects are treated the same on each device. Eg. You could just give global ids as 'Sample1', 'Sample2', etc.

Answer 2

To integrate Drew's answer, I guess the two steps are the following.

1 Implement CDEPersistentStoreEnsemble delegate method (see README)

- (NSArray *)persistentStoreEnsemble:(CDEPersistentStoreEnsemble *)ensemble 
    globalIdentifiersForManagedObjects:(NSArray *)objects {
    return [objects valueForKeyPath:@"yourUniqueIdentifier"];
}

2 Generate the unique identifier for a NSManagedObject subclass

- (void)awakeFromInsert {
    [super awakeFromInsert];

    if (!self.yourUniqueIdentifier) {
        self.yourUniqueIdentifier = [[NSUUID UUID] UUIDString];
    }
}

In awakeFromInsert you can initialize special default property values, like for example an identifier.

The check is necessary, for example, when you have parent-child contexts. Otherwise you are overwriting the identifier previously set. See Why is awakeFromInsert called twice?.