I know you're tired of hearing about NSFetchedResultsController, but…

Okay, there is a known problem with NSFetchedResultsController when using section name key paths. When you make a change to an existing object that results in a new section being created, it crashes. The fetched results controller's delegate never gets notified of a section insert, nor does it get notified about the object moving between sections. It just gets notification that the object was updated.

I've been fighting with NSFetchedResultsController for quite some time now, but this afternoon, I think I made it my bitch. You can actually handle this situation, though it requires a bit of gnarley code. I'm going to update the Navigation-Based Core Data Application Xcode project template shortly with this logic, but here's how you handle this problem in the fetched results controller delegate methods. Specifically look at the NSFetchedResultsChangeUpdate case statement in controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:. There, we determine if the section name key path has changed, and if it has, we determine if we need to add a new section to the table. Then, we fall through tot he NSFetchedResultsChangeMove and follow the same logic.

One caveat, though, I don't know if this will work with section name key paths that use nested keys (e.g. "foo.bar"). In fact, I'm pretty sure it won't work, but I will look at fixing that a little later. For the vast majority of situations, this should patch up the section problem in NSFetchedResultsController pretty well.

The code has been fixed to support nested keys in the section name key path.

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView beginUpdates];
}
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView endUpdates];
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeDelete:
            [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeUpdate: {
            NSString *sectionKeyPath = [controller sectionNameKeyPath];
            NSManagedObject *changedObject = [controller objectAtIndexPath:indexPath];
            NSArray *keyParts = [sectionKeyPath componentsSeparatedByString:@"."];
            id currentKeyValue = [changedObject valueForKeyPath:sectionKeyPath];
            
            // If nested keys, need to get the related object to get
            // the committed key value
            
            for (int i = 0; i < [keyParts count] - 1; i++) {
                NSString *onePart = [keyParts objectAtIndex:i];
                changedObject = [changedObject valueForKey:onePart];
            }
            sectionKeyPath = [keyParts lastObject];
            NSDictionary *committedValues = [changedObject committedValuesForKeys:nil];
            if ([[committedValues valueForKeyPath:sectionKeyPath] isEqual:currentKeyValue])
                break;
            
            NSUInteger tableSectionCount = [self.tableView numberOfSections];
            NSUInteger frcSectionCount = [[controller sections] count];
            if (tableSectionCount != frcSectionCount) {
                // Need to insert a section
                NSArray *sections = controller.sections;
                NSInteger newSectionLocation = -1;
                for (id oneSection in sections) {
                    NSString *sectionName = [oneSection name];
                    if ([currentKeyValue isEqual:sectionName]) {
                         newSectionLocation = [sections indexOfObject:oneSection];
                        break;
                    }
                }
                if (newSectionLocation == -1)
                    return; // uh oh
                
                if (!(newSectionLocation == 0 && tableSectionCount == 1))
                    [self.tableView insertSections:[NSIndexSet indexSetWithIndex:newSectionLocation] withRowAnimation:UITableViewRowAnimationFade];
                NSUInteger indices[2] = {newSectionLocation, 0};
                newIndexPath = [[[NSIndexPath alloc] initWithIndexes:indices length:2] autorelease]; 
            }
        }
        case NSFetchedResultsChangeMove:
            if (newIndexPath != nil) {
                [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
                [self.tableView insertRowsAtIndexPaths: [NSArray arrayWithObject:newIndexPath]
                                      withRowAnimation: UITableViewRowAnimationRight];
            }
            else {
                [self.tableView reloadSections:[NSIndexSet indexSetWithIndex:[indexPath section]] withRowAnimation:UITableViewRowAnimationFade];
            }
            break;
        default:
            break;
    }
}
- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {
    switch(type) {      
        case NSFetchedResultsChangeInsert:
            if (!(sectionIndex == 0 && [self.tableView numberOfSections] == 1))
                [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeDelete:
            if (!(sectionIndex == 0 && [self.tableView numberOfSections] == 1))
                [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
        case NSFetchedResultsChangeMove:
            break;
        case NSFetchedResultsChangeUpdate: 
            break;
        default:
            break;
    }
}