NSFetchedResultsController returns incorrect NSFetchedResultsChangeType

I am creating NSFetchedResultsController

with the following Fetch request inviewDidLoad

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:NSStringFromClass([Person class])];
fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:NSStringFromSelector(@selector(sortDate))
                                                               ascending:NO]];

NSFetchedResultsController *controller = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                             managedObjectContext:self.mainManagedObjectContext
                                                                               sectionNameKeyPath:nil
                                                                                        cacheName:nil];
controller.delegate = delegate;
NSError *error;
if (![controller performFetch:&error]) {
    NSLogError(error);
}

// Connecting up all the delegates for UITableView

      

        
        
        
      

    

And reloading it in viewWillAppear

with[self.tableView reloadData]

Simplified Person

@interface Person : NSManagedObject
/// an NSTimeInterval representing the Unix Epoch
@property (nonatomic, retain) NSNumber *sortDate;
- (void)viewed;
@end

@implementation Person
// nothing fancy or custom - no extra KVO or primitive assignments
@dynamic sortDate;
- (void)viewed {
    self.sortDate = @([NSDate date].timeIntervalSince1970);
}
@end

      

        
        
        
      

    

My data is generated with a network request to fetch the json heap and convert it to NSManagedObject

s. The sort looks ok.

1 Person A time 1437498898
2 Person B time 1437498897
3 Person C time 1437498896

      

        
        
        
      

    

When I set the property sortDate

to Person C

with a method -viewed

, in <NSFetchedResultsControllerDelegate>

I get the change type NSFetchedResultsChangeMove

and the list is sorted like this:

1 Person C time 1437498900
2 Person A time 1437498898
3 Person B time 1437498897

      

        
        
        
      

    

When I try to do the same for Person B

, I instead get the change type NSFetchedResultsChangeUpdate

and the list is sorted like this:

1 Person C time 1437498900
2 Person A time 1437498898
3 Person B time 1437498905

      

        
        
        
      

    

I can constantly update sortDate

for faces A

and C

, and they will continue to sort correctly. Any changes to are Person B

not reflected until the application restarts and then Person B

stops updating. Uninstalling and reinstalling my app (clearing CoreData) doesn't help.

If I put the command print

in - (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath

, then I see that it has Person B

sortDate

indeed updated to the new value, which should force NSFRC to create an event Move

instead of Update

changing the type.

// Original Sort (logs from cellForRowAtIndexPath)
Time 1437506506 - Person B
Time 1437506490 - Person C
Time 1437506381 - Person D
Time 1437506128 - Person A

// Change Person A
Updating SortDate 1437506128 -->  1437506599
Time 1437506599 - Person A for NSFetchedResultsChangeMove

// Sorted Correctly
Time 1437506599 - Person A
Time 1437506506 - Person B
Time 1437506490 - Person C
Time 1437506381 - Person D

// Change Person B
Updating SortDate 1437506506 -->  1437506610
Time 1437506610 - Person B for NSFetchedResultsChangeUpdate

// Sorted Incorrectly
Time 1437506599 - Person A
Time 1437506610 - Person B
Time 1437506490 - Person C
Time 1437506381 - Person D

      

        
        
        
      

    

So why is the NSFetchedResultsController

return wrong NSFetchedResultsChangeType

?