Core Data Objects in Wrong Sections

NSFetchedResultsController is a really handy class. Use one of the default Core Data templates in Xcode and you’ll very quickly have a nice list of managed objects in a table view. With a few more lines of code you can get the NSFetchedResultsController to group your objects by sections. You do this by specifying a key-path in the class’s constructor method but there is another step that if overlooked will cause some confusion.

In a sample app I’ve created a food table that lists food in categories.

FetchedResultsController method grouping sections using a key-path:

Objective-C:

- (NSFetchedResultsController *)fetchedResultsController {
	if (fetchedResultsController != nil) {
		return fetchedResultsController;
	}

	// Create and configure a fetch request with the food entity.
	NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
	NSEntityDescription *entity = [NSEntityDescription entityForName:@"RWFood" inManagedObjectContext:managedObjectContext];
	[fetchRequest setEntity:entity];

	// Create the sort descriptors array.
	NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
	NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:nameDescriptor, nil];
	[fetchRequest setSortDescriptors:sortDescriptors];

	// Create and initialize the fetch results controller.
	NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"category" cacheName:@"Food"];
	self.fetchedResultsController = aFetchedResultsController;
	fetchedResultsController.delegate = self;

	// Memory management.
	[aFetchedResultsController release];
	[fetchRequest release];
	[nameDescriptor release];
	[sortDescriptors release];

	return fetchedResultsController;
}

Swift:

lazy var fetchedResultsController: NSFetchedResultsController<rwfood> = {
    let fetchRequest: NSFetchRequest<rwfood> = RWFood.fetchRequest()
    let sortDescriptor = NSSortDescriptor(key: "name", ascending: true)
    fetchRequest.sortDescriptors = [sortDescriptor]
    let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: "category", cacheName: "Food")
    aFetchedResultsController.delegate = self
    do {
        try aFetchedResultsController.performFetch()
    } catch let error {
        print("Unable to perform fetch: \(error)")
    }
    return aFetchedResultsController
}()

Specify a key-path

Save and quit the app a few times and you’ll see the objects seem to be in the wrong sections. If you look closer you’ll see that the objects are actually sorted in ascending name order. On looking at the code, it seems this is exactly what we asked the program to do! After some testing it also seems to show up more often if the table is a grouped one.

As per the docs, after you specify a key-path to group each section with you also need to make sure the first sort descriptor is sorting this key-path. Add a sort descriptor and everything will work as expected.

Revised fetchedResultsController method with missing sort descriptor:

Objective-C:

- (NSFetchedResultsController *)fetchedResultsController {

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

	// Create and configure a fetch request with the plant entity.
	NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
	NSEntityDescription *entity = [NSEntityDescription entityForName:@"RWPlant" inManagedObjectContext:managedObjectContext];
	[fetchRequest setEntity:entity];

	// Create the sort descriptors array.
	NSSortDescriptor *typeDescriptor = [[NSSortDescriptor alloc] initWithKey:@"type" ascending:YES];
	NSSortDescriptor *nameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
	NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:typeDescriptor, nameDescriptor, nil];
	[fetchRequest setSortDescriptors:sortDescriptors];

	// Create and initialize the fetch results controller.
	NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:@"type" cacheName:@"Plants"];
	self.fetchedResultsController = aFetchedResultsController;
	fetchedResultsController.delegate = self;

	// Memory management.
	[aFetchedResultsController release];
	[fetchRequest release];
        [categoryDescriptor release];
	[nameDescriptor release];
	[sortDescriptors release];

	return fetchedResultsController;
}

Swift:

lazy var fetchedResultsController: NSFetchedResultsController = {
    let fetchRequest: NSFetchRequest<rwplant> = RWPlant.fetchRequest()
    let typeDescriptor = NSSortDescriptor(key: "type", ascending: true)
    let nameDescriptor = NSSortDescriptor(key: "name", ascending: true)
    fetchRequest.sortDescriptors = [typeDescriptor, nameDescriptor]
    let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: managedObjectContext, sectionNameKeyPath: "type", cacheName: "Plants")
    aFetchedResultsController.delegate = self
    do {
        try aFetchedResultsController.performFetch()
    } catch let error {
        print("Unable to perform fetch: \(error)")
    }
    return aFetchedResultsController
}()