Move object from one Core Data UITableView to another?

Question

I'm trying to move an object from a fetchedresultscontroller to another. The code looks like this:

UISegmentedControl *segmentedControl = self.navigationItem.titleView;
NSManagedObject *managedObject = [fetchedResultsController objectAtIndexPath:indexPath];
NSDate *timeValue = [[managedObject valueForKey:@"timeStamp"] copy];

NSManagedObjectContext *context = [fetchedResultsController managedObjectContext];
[context deleteObject:[fetchedResultsController objectAtIndexPath:indexPath]];

NSError *error = nil;
if (![context save:&error]) {
    /*
     Replace this implementation with code to handle the error appropriately.

     abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
     */
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}
[self.tableView reloadData];

fetchedResultsController = [fetchedResultsControllers objectAtIndex:(1-segmentedControl.selectedSegmentIndex)];
context = [fetchedResultsController managedObjectContext];
NSManagedObject *newManagedObject = [NSEntityDescription insertNewObjectForEntityForName:@"Picked" inManagedObjectContext:context];

[newManagedObject setValue:timeValue forKey:@"timeStamp"];

error = nil;
if (![context save:&error]) {
    /*
     Replace this implementation with code to handle the error appropriately.

     abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button.
     */
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

fetchedResultsController = [fetchedResultsControllers objectAtIndex:(1-segmentedControl.selectedSegmentIndex)];

}

But it doesn't work. I get the dreaded:

Serious application error.  Exception was caught during Core Data change processing: Invalid update: invalid number of rows in section 0.  The number of rows contained in an existing section after the update (1) must be equal to the number of rows contained in that section before the update (2), plus or minus the number of rows inserted or deleted from that section (1 inserted, 0 deleted). with userInfo (null)

I don't really know how to get it to work. Core Data is hard to work with.

Answer 1

I ran into this problem myself last night. In my case, I had forgotten to implement the NSFetchedResultsControllerDelegate methods:

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath;
- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type;
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller;
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller;

The problem goes something like this: after you delete an object in the NSManagedObjectContext, and then save the context, the UITableView is out of sync with the changes and is asking for items that don't exist.

The CoreDataBooks sample code shows how to implement these functions in conjunction with adding/deleting managed objects in your NSFetchedResultsController.