NSFetchedResultsController: Two sort descriptors with the same key not working. One descending for sections, the other ascending for rows

Question

I use a NSFetchedResultsController for populating a TableView. I want the sections to be sorted descending by date and the rows sorted ascending by date. Both use the key 'creation' and the sections have a transient property for the sectionNameKeyPath which returns a string for the date with the form: "Today, Yesterday, 20.11.2013, 19.11.2013, …" created from the 'creation' date. However, the rows are always sorted in the ordering of the first sort descriptor. Is this approach wrong?

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Exercise"];
request.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"creation" ascending:NO],
                            [NSSortDescriptor sortDescriptorWithKey:@"creation" ascending:YES]];

self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                    managedObjectContext:self.context
                                                                      sectionNameKeyPath:@"localizedCreationDate"
                                                                               cacheName:nil];

I am grateful for any provided help! br

UPDATE

The above approach can't work. :) I solved it by saving a second date as a property with the beginning of the day, ordered descending. This is also used as the sectionNameKeyPath. Additionally, for section header titles the localizedCreationDate is used.

- (void)setuptFetchedResultsController
{
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"DNALoggedExercise"];
    request.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"creationDayDate" ascending:NO],
                                [NSSortDescriptor sortDescriptorWithKey:@"creationDate" ascending:YES]];

    self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                        managedObjectContext:self.context
                                                                          sectionNameKeyPath:@"creationDayDate"
                                                                                   cacheName:nil];
}

Jean-Étienne Jean-Étienne · Accepted Answer · 2013-12-29T02:56:46

In your first piece of code, you're giving two contradictory NSSortDescriptors, so only one is considered, and probably the first.

By creating a second date property just for convenience, you're introducing data duplication in your database, that's not a very good idea. Why not sorting your results in whichever order, and just reading the array in reverse in cellForRowAtIndexPath or titleForHeaderInSection ? Even better, why not having a small private method in your viewController that would prepare a NSDictionary or any other convenience data structure for later use?

NSFetchedResultsController: Two sort descriptors with the same key not working. One descending for sections, the other ascending for rows

1 Answers