Updating core data entry

Question

I'm working with core data to save scores for a game, and need help updating an entry when a score is beaten. This is where I'm at. I write the data the first time the app launches like this:

LevelData *levelOne = [NSEntityDescription insertNewObjectForEntityForName:@"LevelData" inManagedObjectContext:context];
    levelOne.levelNum = @"1";
    levelOne.topScore = @"0";
    levelOne.isPassed = @"No";

    if (![context save:&error]) {
        NSLog(@"coudlnt save: %@", [error localizedDescription]);
    }

I then read out the data at the end of the level like this:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"levelNum == 1"];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"LevelData" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSError *error;
NSArray *fetchedObjects = [managedObjectContext executeFetchRequest:fetchRequest error:&error];
for (LevelData *info in fetchedObjects) {
    NSLog(@"Is Passed: %@", info.levelNum);
    NSLog(@"Top Score: %@", info.topScore);
    NSLog(@"Is Passed: %@", info.isPassed);
}

What I'm stuck doing is updating the topScore entry and writing it back to the data store without creating a new entry, but updating the existing one.

And help/example would be so so helpful and much appreciated.

Thanks, Kyle

score 0 · Answer 1 · answered Aug 21 '12 at 15:06

Well, updating the entry from the data store is easy. Fetch, update, save. As you seem to have a handle on. The real issue with core data is defining that initial data store. The design of core data does not bring in the concept of a database, so typically, one does not exist until the backend implements it for you (after your application calls its first save).

What I have seen developers typically do to work around this is set up AppDelegate to implement a quick and dirty hack: save an empty data base. They then comment out this code; retrieve the file from disk, rename it, modify it, add it to the project's support files.

Now, revisiting AppDelegate, they test on application launch the existence of the datastore on the client's machine. If none exists, they know to inject the default data store from their support files.. basically copying it to the appropriate path and renaming it appropriately as well.

This gets overly complicated with versioning. Another approach to this is to write a plist of the defaults into the main bundle and import this on the first launch and store one key in user defaults to let me know if this is necessary. It requires a little more boilerplate code, but it is a great way to get things set up. And you can encode versioning data to support upgrade merging a lot easier. If you can handle core data well, manipulating XML shouldn't be much more difficult.

Thanks for your reply, I have the backend setup and working how I want. The problem is whenever I try and save, I just seem to end up adding a new entry to the data store, thus resulting in multiple entries for levelNum 1. So if you could show me any code to update and save based on the code I am already using that would be amazing. — Kyle Goslan, Aug 21 '12 at 15:19

Updating core data entry

1 Answers1