Problems after adding NSFetchedResultsController to a project - ios

Question

I set up CoreData without NSFetchedResultsController and had everything saving fine. After switching to NSFetchedResultsController, I am getting a strange error when trying to save the image.
Here is the code I am using to save the image:
- (void)saveImage {
NSManagedObjectContext *context = [self managedObjectContext];
TimeTravelFeed *timeTravelFeed = [NSEntityDescription insertNewObjectForEntityForName:#"TimeTravelFeed" inManagedObjectContext:context];
NSData *imageData = UIImageJPEGRepresentation(self.thumbImage, 0.8f);
[timeTravelFeed setValue:imageData forKey:#"imageData"];
NSError *error = nil;
if (![self.managedObjectContext save:&error]) {
NSLog(#"Unresolved error %#, %#", error, [error userInfo]);
abort();
}
//[tableView reloadData];
}
And here is the error message:
-[_PFExternalReferenceData compare:]: unrecognized selector sent to instance 0x1669fb40
2013-12-08 10:09:49.442 Time Travel[830:60b] CoreData: error: Serious application error. Exception was caught during Core Data change processing. This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification.
-[_PFExternalReferenceData compare:]: unrecognized selector sent to instance 0x1669fb40 with userInfo (null)
2013-12-08 10:09:49.443 Time Travel[830:60b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[_PFExternalReferenceData compare:]: unrecognized selector sent to instance 0x1669fb40'
Here is the code for NSFetchedResultsController:
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
DetailViewController *detailsViewController = [[DetailViewController alloc] init];
[self.navigationController pushViewController:detailsViewController animated:YES];
}
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
[self.tableView beginUpdates];
}
- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {
switch(type) {
case NSFetchedResultsChangeInsert:
[self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex]
withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeDelete:
[self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex]
withRowAnimation:UITableViewRowAnimationFade];
break;
}
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
newIndexPath:(NSIndexPath *)newIndexPath {
UITableView *tableView = self.tableView;
switch(type) {
case NSFetchedResultsChangeInsert:
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath]
withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeDelete:
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath]
withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeUpdate:
[self configureCell:[tableView cellForRowAtIndexPath:indexPath]
atIndexPath:indexPath];
break;
case NSFetchedResultsChangeMove:
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath]
withRowAnimation:UITableViewRowAnimationFade];
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath]
withRowAnimation:UITableViewRowAnimationFade];
break;
}
}
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
[self.tableView endUpdates];
}
- (NSFetchedResultsController *)fetchedResultsController {
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription
entityForName:#"TimeTravelFeed" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *sort = [[NSSortDescriptor alloc]
initWithKey:#"imageData" ascending:NO];
[fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
[fetchRequest setFetchBatchSize:20];
NSFetchedResultsController *theFetchedResultsController =
[[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:managedObjectContext sectionNameKeyPath:nil
cacheName:#"Root"];
self.fetchedResultsController = theFetchedResultsController;
_fetchedResultsController.delegate = self;
return _fetchedResultsController;
}

Answer 1

The problem seems to be here:
NSSortDescriptor *sort = [[NSSortDescriptor alloc]
initWithKey:#"imageData" ascending:NO];
You cannot sort on a "Binary Data" attribute. A fetched results controller
needs a sort descriptor, so you should use a different attribute, e.g. a string, number or date. For example
NSSortDescriptor *sort = [[NSSortDescriptor alloc]
initWithKey:#"creationDate" ascending:NO];
where "creationDate" is an attribute (type "Date") and set when the object is created:
[timeTravelFeed setValue:imageData forKey:#"imageData"];
[timeTravelFeed setValue:[NSDate date] forKey:#"creationDate"];

Answer 2

Go to your DBfile, select entity named "TimeTravelFeed" and select key "imageData". Set properties.