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;
}
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.