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 obje

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  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.