Question

situation: CoreData db with “contacts” entities and “Sum” attribute (positive or negative or 0)

goal: tableviewcontroller with 3 sections, 1st positive 2nd negative 3rd (zero) “ARCHIVED”

so far: sort descrpitor with @selector(compare:) (without predicate because list of all contacts wanted)

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Contact"];

request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"moneySum" ascending:YES selector:@selector(compare:)]];
self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                    managedObjectContext:self.database.managedObjectContext
                                                                      sectionNameKeyPath:@"sectionName"
                                                                               cacheName:nil];

and in a Category for Contact Entity:

- (NSString *)sectionName {

    [self willAccessValueForKey:@"sectionName"];
    NSString *sectionName;
    double value = [[self valueForKey:@"moneySum"] doubleValue];

    if      (value > 0) sectionName = POS_SECTION;
    else if (value < 0) sectionName = NEG_SECTION;
    else                sectionName = ARCHIVE_SECTION;

    [self didAccessValueForKey:@"sectionName"];
    return sectionName;
}

I do get 3 categories now, but it obviously sorts them with archive in the middle, not at the end.

I thought about adding a second sortDescriptor to the sortDescriptors array (before compare: one) but the most intuitive way for me (isEqualToNumber:) obviously doesn’t work because i can’t specify any arguments (@selector ( ..:) ) or am I wrong?

Sorry, quite new to this whole coding thing 🙂

Answer 1

From the “Core Data Programming Guide”:

… To summarize, though, if you execute a fetch directly, you should
typically not add Objective-C-based predicates or sort descriptors to
the fetch request. Instead you should apply these to the results of
the fetch.

This means that you cannot use a custom sort descriptor in your fetch request. You must store an additional (non-transient) attribute in the entity, for example “0”, “1”, “2” for positive/negative/zero sums.

You can then use this attribute for both the sort descriptor and for the sectionNameKeyPath, you don’t need a transient attribute “sectionName”.

The mapping from “0”, “1”, “2” to the actual section header is then done in tableView:titleForHeaderInSection:.

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section { 
    id <NSFetchedResultsSectionInfo> sectionInfo = [[self.controller sections] objectAtIndex:section];
    int order = [[sectionInfo name] intValue];
    if (order == 0)
        return @"POSITIVE SECTION";
    else if (order == 1)
        return @"NEGATIVE SECTION";
    else
        return @"ARCHIVE SECTION";
}