iPhone SDK Core Data: Fetch all entities with a nil relationship?
- by Harkonian
I have a core data project that has Books and Authors. In the data model Authors has a to-many relationship to Books and Books has a 1-1 relationship with Authors. I'm trying to pull all Books that do not have an Author. No matter how I try it, no results are returned. In my predicate I've also tried = NIL, == nil, == NIL. Any suggestions would be appreciated.
// fetch all books without authors
- (NSMutableArray *)fetchOrphanedBooks {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Book" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
[fetchRequest setFetchBatchSize:20];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"author = nil"];
[fetchRequest setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
NSString *sectionKey = @"name";//nil;
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext
sectionNameKeyPath:sectionKey cacheName:nil];
BOOL success = [aFetchedResultsController performFetch:nil];
NSMutableArray *orphans = nil;
// this is always 0
NSLog(@"Orphans found: %i", aFetchedResultsController.fetchedObjects.count);
if (aFetchedResultsController.fetchedObjects.count > 0)
{
orphans = [[NSMutableArray alloc] init];
for (Note *note in aFetchedResultsController.fetchedObjects)
{
if (note.subject == nil)
{
[orphans addObject:note];
}
}
}
[aFetchedResultsController release];
[fetchRequest release];
[sortDescriptor release];
[sortDescriptors release];
return [orphans autorelease];
}
