iPhone How to modify the data contained on the Persistent store using Core Data

Question

Hello,

I'm stuck trying to figure out how to modify the data contained on the persistent store. I'm writing an application with several views using a tabBarController, my core data methods are located mainly on the main application delegate but I will only be using this data from the tableViewController view.

In order to use the managedObjectContext created in the main application delegate from the tableViewController I use the following on the viewDidLoad method:

MessageAppDelegate *appDelegate = (MessageAppDelegate *)[[UIApplication sharedApplication] delegate];
managedObjectContext = [appDelegate managedObjectContext];

The application then displays some messages in the table and when a user selects a UITableViewCell (didSelectRowAtIndexPath) I get the ID of the message object and call the following method:

[self readMessage:pk];
-(void)readMessage:(NSInteger)pk {  
// First I select the data
NSFetchRequest *request = [[NSFetchRequest alloc] init];
// had to setReturnsObjectsAsFaults to NO so I could access the message data
[request setReturnsObjectsAsFaults:NO];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Message" inManagedObjectContext:self.managedObjectContext];
[request setEntity:entity];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"pk == %d", pk];
[request setPredicate:predicate];

NSError *error;
NSArray *items = [self.managedObjectContext executeFetchRequest:request error:&error];
[request release];

// Then I update the object
for (Message *thisMessage in items) {
    //I display the message to the console before updating to check the value
    DLog(@"before reading message %@", thisMessage);
    // we set the message flat to YES
    [thisMessage setRead:YES];
    // we set some sample text here (just for testing)
    [thisMessage setMessageText:@"New message text"];
    // I then display the message to the console checking that the flag and text has been updated
    DLog(@"read message %@", thisMessage);
}

// Finally I save the updated message calling the function posted below
[self saveMOC];
}

- (void)saveMOC {
 NSError *error;
 if (![managedObjectContext save:&error]) {
  NSLog(@"there was an error saving the message!");
 }
}

After that the data gets updated correctly and if I fetch the data from the managedObjectContext after saving it I get the correct values. I verified this by adding the following code to at the end of readMessage method:

    request = [[NSFetchRequest alloc] init];
//required to avoid presenting objects as faults!!
[request setReturnsObjectsAsFaults:NO];
entity = [NSEntityDescription entityForName:@"Message" inManagedObjectContext:[self managedObjectContext]];
[request setEntity:entity];


//Set the sort descriptor
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"pk" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[request setSortDescriptors:sortDescriptors];
[sortDescriptors release];
[sortDescriptor release];

//Execute the request
NSMutableArray *mutableFetchResults = [[self.managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
if (mutableFetchResults == nil) {
    // Handle the error later
    DLog(@"ERROR: Unable to fetch the results");
}

[self setMessagesArray:mutableFetchResults];
    NSLog(@"Data now is: %@", mutableFetchResults);
[mutableFetchResults release];
[request release];

The problem is that if I exit from the application and launch it again all my messages lose the read property (or any other changes I make) and the tableview loads the data as it was first saved onto the persistent store.

Any help would be highly appreciated!! (-_-)

Answer 1

Try this and see if the object changes are actually being saved

- (void)saveMOC {
 NSError *error;
 if (![managedObjectContext save:&error]) {
  NSLog(@"there was an error saving the message!");
 } else {
  NSLog(@"The message was saved!");
 }
}

So for each call of saveMOC which is successful, you should see a console message. If it is being called and you're seeing the messages, then you must not be altering the 'read message' property. You could check this by inspecting the value of the 'read message' property before and after setting it either using a breakpoint or by using NSLog messages to print its value

Answer 2

Is -readMessage: method defined in your app delegate or in your view controller? My guess is that you're changing properties of an object in different managed object context than one where you try to save changes (MOC in your app delegate), which actually doesn't have an idea that something has changed. On the other hand, MOC which keeps your changes is never saved (changes are kept only in memory) and that for your changes are lost after you restart your app.

Can this be the situation?