Custom Core Data SectionNameKeyPath

core-data, ios, sections, transient

Solution

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.

Problem

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; } ```

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