exception thrown trying to insert new table row into section

Question

I seem to be encountering a very odd problem which only occurs when trying to insert a new NSManagedObject into a new section. Basically my sections are days and individual cells are associated with times. When I MOVE an object to a day which doesn't currently have another object (table row) associated with the day, the table needs to create a new section and move the row. Instead of this happening properly I'm getting the following:

Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. * -[NSMutableArray removeObjectAtIndex:]: index 1 beyond bounds [0 .. 0] with userInfo (null)

Inserting a new object for a day that currently has no other objects seems to work fine.

The problem seems to be somewhere in this code, but I can't seem to figure out what's wrong with it. controller:didChangeSection:... seems to get called first with an insert and a delete then controller:didChangeObject:... is called with NSFetchedResultsChangeMove. So the order is:

NSFetchedResultsChangeInsert (in didChangeSection) NSFetchedResultsChangeDelete (in didChangeSection) NSFetchedResultsChangeMove (in didChangeObject)

/**
 Delegate methods of NSFetchedResultsController to respond to additions, removals and so on.
 */

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
// The fetch controller is about to start sending change notifications, so prepare the table view for updates.
[self.plainTableView beginUpdates];
}

- (void)controller:(NSFetchedResultsController *)controller
   didChangeObject:(id)anObject
   atIndexPath:(NSIndexPath *)indexPath
 forChangeType:(NSFetchedResultsChangeType)type
  newIndexPath:(NSIndexPath *)newIndexPath {
UITableView *tv = self.plainTableView;

switch(type) {
    case NSFetchedResultsChangeInsert:
        [tv insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeDelete:
        [tv deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeUpdate:
        [self configureCell:(UITableViewCell *)[tv cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
        break;

    case NSFetchedResultsChangeMove:
        [tv deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        // Reloading the section inserts a new row and ensures that titles are updated appropriately.
        [tv reloadSections:[NSIndexSet indexSetWithIndex:newIndexPath.section] withRowAnimation:UITableViewRowAnimationFade];
        break;
}
}

- (void)controller:(NSFetchedResultsController *)controller
  didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
           atIndex:(NSUInteger)sectionIndex
     forChangeType:(NSFetchedResultsChangeType)type {
    switch(type) {
    case NSFetchedResultsChangeInsert:
        [self.plainTableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
        break;

    case NSFetchedResultsChangeDelete:
        [self.plainTableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
        break;
}
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
// The fetch controller has sent all current change notifications, so tell the table view to process all updates.
[self.plainTableView endUpdates];
}

I thought this was all just standard template code though. Does anyone have any idea what might be wrong?

Actually it also happens when trying to insert a new row into a day which already exists, but not when inserting a new row/object for a day that doesn't already exist.

Answer 1

I seem to have fixed this by copying the following code from the latest version of CoreDataBooks:

    case NSFetchedResultsChangeMove:
        [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
        break;

Before that the code here was:

    case NSFetchedResultsChangeMove:
        [tv deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
        // Reloading the section inserts a new row and ensures that titles are updated appropriately.
        [tv reloadSections:[NSIndexSet indexSetWithIndex:newIndexPath.section] withRowAnimation:UITableViewRowAnimationFade];
        break;

Answer 2

I think you have a conceptual problem here.

The controller:didChange... methods are supposed to called in response to a change in the data model as detected by the fetched results controller (FRC). I'm pretty sure your problem here is that you are changing the table before or without changing the underlying data.

Sections and rows are not attributes of the table, they are attributes of the data as structured by the FRC. If you need to add a section, you need to add it first to the data and then let the FRC delegate methods bring the table up to date. The table reflects the data model and not the other way around.

If you do not do it this way, the FRC will report the wrong number of sections or rows and cause the table to crash with the error you saw.