Core Data Sort Descriptors with relationships between Entities

I have two entities, Job and Client. The relationship from job to client is called clientOfJob which is a many-to-one relationship:

Job<<-->Client

I am trying to understand how I would sort my fetch results into sections for the table view, with the sections based on client first names.

This is my first attempt to use sectionNameForKeyPath, but I am using sample code from CoreDataBooks, and so far, substituting some of that code has eliminated some serious problems I had earlier today.

Here is the code I tried. I didn't think it would work, but I'm not sure what I need to use for the client sort descriptor key and for the sectionNameKeyPath.

By the way, the code works well with only the jobSortDescriptor (without the clientSortDescriptor) and with sectionNameKeyPath set to nil. Then it just sorts by job description. I can get at the related client entity information, putting client name information into the detailedTextLabel of each cell while I have the job description in the textLabel of the cell.

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

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription * entity = [NSEntityDescription entityForName:@"Job" inManagedObjectContext:dataInterface.managedObjectContext];

    [fetchRequest setEntity:entity];

    [fetchRequest setFetchBatchSize:10];

    NSSortDescriptor *clientSortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"clientOfJob.firstName" ascending:YES];
    NSSortDescriptor *jobSortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"jobDescription" ascending:YES];

    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:clientSortDescriptor, jobSortDescripto开发者_JS百科r, nil];
    [fetchRequest setSortDescriptors:sortDescriptors];

    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:dataInterface.managedObjectContext sectionNameKeyPath:@"clientOfJob.firstName" cacheName:@"Jobs"];

    aFetchedResultsController.delegate = self;

    self.fetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [fetchRequest release];
    [jobSortDescriptor release];
    [clientSortDescriptor release];
    [sortDescriptors release];

    NSError *error = nil;

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