Custom Core Data SectionNameKeyPath

Question

I am new at core data and am trying to figure out how to create a custom sectionNameKeyPath in my NSFetchedResultsController. I have a managed object with an attribute called acctPeriod. It is a NSString. I want to create sections based on the first 4 characters of this field. The first 4 characters represent the year of the accounting period and doesn't need to be saved.

I have gone through this site and have seen posts about transient attributes but I can't seem to get them to work. Basically I want this and then assign periodYear for my sectionNameKeyPath.

@dynamic periodYear;

-(NSString *)periodYear
{
    return [self.acctPeriod substringToIndex:4];
}

Any help would be appreciated.

**UPDATE: Using Martin R. answer, I was able to get it to work as expected.

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

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];

// Edit the entity name as appropriate.
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Billing" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];

// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:20];

// Edit the sort key as appropriate.
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"acctPeriod" ascending:NO];
NSArray *sortDescriptors = @[sortDescriptor];

//Predicate
NSPredicate *pred = [NSPredicate predicateWithFormat:@"clients = %@", self.client];
NSLog(@"%@",pred);

//[fetchRequest setResultType:NSDictionaryResultType];
//[fetchRequest setReturnsDistinctResults:YES];

[fetchRequest setPredicate:pred];
[fetchRequest setSortDescriptors:sortDescriptors];

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"periodYear" cacheName:nil];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;

NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error])
{
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
}

return _fetchedResultsController;  
}

Answer 1

The following should work: Implement the periodYear method (which will be used as "section name key path") in a class extension of your managed object subclass:

@interface Event (AdditionalMethods)
- (NSString *)periodYear;
@end

@implementation Event (AdditionalMethods)
- (NSString *)periodYear {
    return [self.acctPeriod substringToIndex:4];
}
@end

Make sure that acctPeriod is used as the first (or only) sort descriptor for the fetch request:

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"acctPeriod" ascending:YES];
NSArray *sortDescriptors = @[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];

Use periodYear as sectionNameKeyPath for the fetched results controller:

NSFetchedResultsController *_fetchedResultsController = [[NSFetchedResultsController alloc]
                  initWithFetchRequest:fetchRequest 
                   managedObjectContext:self.managedObjectContext 
                     sectionNameKeyPath:@"periodYear"
                              cacheName:nil];
_fetchedResultsController.delegate = self;
self.fetchedResultsController = _fetchedResultsController;

And finally add the default titleForHeaderInSection method:

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section {
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo name];
}

Alternatively, you can define periodYear as transient attribute of the managed object. It will also not be stored in the database in that case, but can be implemented in a way that the value is calculated on demand and cached.

The DateSectionTitles sample project from the Apple Developer Library demonstrates how this works.