iphone - NSFetchedResultsController custom sort not getting called

Question

I am currently trying to populate a UITableView in my project from Core Data using NSFetchedResultsController. I am using a custom search with a comparator (although I have also tried a selector and had an identical problem):

    if (fetchedResultsController != nil) {
        return fetchedResultsController;
    }

    /*
     Set up the fetched results controller.
    */
    // Create the fetch request for the entity.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"objectName" ascending:YES comparator:^(id s1, id s2) {
            NSLog(@"Comparator");
      //custom compare here with print statement
    }];
    NSLog(@"Sort Descriptor Set");
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Object" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];
    [fetchRequest setSortDescriptors:sortDescriptors];

    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"firstLetterOfObject" cacheName:@"Objects"];
    [aFetchedResultsController release];
    [fetchRequest release];
    [sortDescriptor release];
    [sortDescriptors release];
    if (![fetchedResultsController performFetch:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return fetchedResultsController;

When I enter this tab, I have logged all over the program and found that the NSFetchedResultsController does not even enter the comparator block when fetching. It instead sorts it with some default sorting method.

If I delete and add an Object with an objectName, however, it then does enter the comparator block and correctly sort the table.

Why does the NSFetchedResultsController not sort using the comparator until the managed object model is changed?

Notes: I have tried also turning off caching, and/or performing a fetch in viewDidLoad:, but it seems that how many times I fetch does not matter, but when. For some reason it only uses my sorting after the object model has been changed.

Answer 1

There are a couple of things I can think of. First, though this may not be your problem, you cannot sort on transient properties. But more likely is that when sorting in a model backed by a SQL store, the comparator gets "compiled" to a SQL query, and not all Objective-C functions are available. In this case, you'd need to sort in memory after the fetch is performed.

EDIT: See this doc, specifically the Fetch Predicates and Sort Descriptors section.