Question

I am using core data in my iPhone app. I have created a simple class Friend, which derives from NSManagedObject and which uses the following property:

@property (nonatomic, retain) NSString *name;

I am able to add and delete instances of this class to my context and my changes are persistent also.
Now I want to update/modify a Friend-instance and make it persistent again.

But this seems not to work.

Here is piece of code which shows my problem:

//  NSManagedObjectContext *context  = < my managed context> 
//   NSFetchedResultsController *nsfrc= < my fetched result controller>

NSEntityDescription *entity = [nsfrc entity];
NSManagedObject *newManagedObject = [NSEntityDescription 
       insertNewObjectForEntityForName:[entity name]  inManagedObjectContext:context];

Friend *f = (Friend *) newManagedObject;
f.name = @"name1";
//1.  --- here context.hasChanges == 1 ---   ok

NSError *error = nil;
if (![context save:&error]) { ... }
//2.  --- here context.hasChanges == 0 ---   ok

f.name = @"name2";
//3.  --- here context.hasChanges == 0 ---   nok?

if (![context save:&error]) { ... }

At comment 1 everything is fine. I got a new NSManagedObject of the Friend-type and I can change the name property. The context shows me that there is something to save.
After saving the context I see context.hasChanges == 0. Note also that the data is persistent after saving the context.

After comment 2 I change the name property. Now I would expect context.hasChanges == 1 and also after the context save I would expect the new name to be persistent.
But unfortunately It is not. Starting the application again, loads the Friend instance with the name-property = @”name1″.

I cannot find any hint or example inside the core data documentation. So what am I doing wrong?
What do I have to do to update/modify an existing Friend-instance and to make it persistent?

The only solution I see is to delete the entry, change it, and add it again. But I don’t think that this is the correct way for it.

Thanks!

Answer 1

Finally found the solution! For my above question I removed all unnessesary code (unfortunately I also removed my problem). So, to complete this thread:
My Friend-class also has a member:

@property (nonatomic) int duration;

In my test environment I also set the duration (whenever I set the name). This seems to be the reason why the context is not able to recognize any change. If I change the property to

@property (nonatomic, retain) NSNumber duration;

everything worked fine. Note that my xdatamodel has a property duration of type Int32.
What I not understand is, why everything worked for the first [context save];. But this is ok for me for now.

To give some good advice here:

Do NOT generate your xdatemodel out of your NSManagedObject-derived class (as I did).
Instead create your xdatemodel and then generate your class out of your model (Xcode->Design->DataModel->Copy Obj-C2.0..)!
(Update 03/2011: Generating the Classes out of the Model works like a charm in XCode 4.0!)

I wish I had known this early on this beautiful sunday morning.
Good evening!