Sort master data with Predicate to eliminate duplicates

Question

I have an event database loaded in Core Data that has duplicate event headers. This is done so that the database can provide unique information for each day of the event. For example, price fluctuations for each date.

Now I need to remove duplicate event names from the list, which will be displayed as a table using NSFetchRequest and NSPredicate to provide a filter. But all the examples that I saw do not require the use of a dynamic key value as a target for a predicate filter. for example, below NSDate provides time now as a key filter, and it works.

Currently, the NSString * header sets a value in the ManagedObject event class, which returns nil. Here is a snippet from the FetchResultsController.

- (NSFetchedResultsController *)fetchedResultsController {
    if (fetchedResultsController == nil) {
        NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
        NSPredicate *predicate = [[[NSPredicate alloc] init] autorelease];
        [fetchRequest setReturnsObjectsAsFaults:NO];  
        [fetchRequest setEntity:[NSEntityDescription entityForName:@"Event" inManagedObjectContext:managedObjectContext]];
        NSArray *sortDescriptors = nil;
        NSString *sectionNameKeyPath = nil;
        NSDate *date = [NSDate date];
        NSString *title = [events title];
        if ([fetchSectioningControl selectedSegmentIndex] == 1) {
            predicate = [NSPredicate predicateWithFormat:@"(closeDate >= %@) AND (title == %@)", date, title ];
            sortDescriptors = [NSArray arrayWithObjects:[[[NSSortDescriptor alloc] initWithKey:@"category.name" ascending:YES] autorelease], [[[NSSortDescriptor alloc] initWithKey:@"openDate" ascending:YES] autorelease], nil];
            sectionNameKeyPath = @"category.name";
        } else if ([fetchSectioningControl selectedSegmentIndex] == 0){
            predicate = [NSPredicate predicateWithFormat:@"closeDate >= %@", date];
            sortDescriptors = [NSArray arrayWithObject:[[[NSSortDescriptor alloc] initWithKey:@"openDate" ascending:YES selector:@selector(compare:)] autorelease]];
            sectionNameKeyPath = @"day"; 
        }
        [fetchRequest setPredicate:predicate];
        [fetchRequest setSortDescriptors:sortDescriptors];
        fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:sectionNameKeyPath cacheName:@"EventsCache"];
    }    
    return fetchedResultsController;
}

Answer 1

You can set

setReturnsDistinctResults:YES

on your fetchRequest.

See the docs for more details: NSFetchRequest class reference