How to switch UITableView's NSFetchedResultsController (or its predicate) programmatically?

cocoa-touch, core-data, master-detail, nsfetchedresultscontroller, uitableview

Solution

After I posted my question, I tried a variant of the code the OP of the other question showed. It works for me. Here it is:

- (void) displaySelection:(Selection *)aSet
{
    if (aSet != self.currentSelection) {
        self.currentSelection = aSet;

        NSFetchRequest *fetchRequest = [[self fetchedResultsController] fetchRequest];
        NSPredicate *predicate = nil;
        NSEntityDescription *entity = nil;
        entity = [NSEntityDescription entityForName:@"DocInSelection" inManagedObjectContext:managedObjectContext];
        predicate = [NSPredicate predicateWithFormat:@"selection.identifier like %@", currentSelection.identifier];
        [fetchRequest setEntity:entity];
        [fetchRequest setPredicate:predicate];

        [NSFetchedResultsController deleteCacheWithName:@"Root"];

        NSError *error = nil;
        if (![[self fetchedResultsController] performFetch:&error]) {
            NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
            abort();
        }       
    }
    [self.tableView reloadData];
}

Problem

I have a UITableView that displays a subset of a large number of entities named "Documents". The subset is defined by another entity "Selection". Selections are named, ordered list of documents. It Works fine, except when I want to change the displayed selection at run time. I get only a blank list. Basically, I need to change the predicate that my NSFetchedResultsController holds so that the new predicate uses the another Selection. I couldn't make it work. My last attempt is to get rid of the NSFetchedResultsController altogether and reallocate it: ``` - (void) displaySelection:(Selection *)aSet { self.currentSelection = aSet; self.fetchedResultsController = nil; // methods here don't all use the property but directly the ivar, so we must trigger the getter [self fetchedResultsController]; [self.tableView reloadData]; } ``` And of course, the NSFetchedResultsController getter does the right thing: ``` - (NSFetchedResultsController *)fetchedResultsController { if (fetchedResultsController != nil) { return fetchedResultsController; } NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"DocInSelection" inManagedObjectContext:managedObjectContext]; [fetchRequest setEntity:entity]; NSPredicate *predicate = [NSPredicate predicateWithFormat:@"selection.identifier like %@", currentSelection.identifier]; [fetchRequest setPredicate:predicate]; <snip> [fetchRequest setSortDescriptors:sortDescriptors]; NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"]; aFetchedResultsController.delegate = self; self.fetchedResultsController = aFetchedResultsController; <snip> return fetchedResultsController; } ``` This code works the first time, because the Initial Selection is set. When `displaySelection:` is called, though, the tableview becomes blank. A very similar question was asked at NSFetchedResultsController fetch request - updating predicate and UITableView And the answer was to get rid of the NSFetchedResultsController. I don't want to do that, because NSFetchedResultsController brings a lot of useful goodies here (eg caching, partial loading...). The question still stands: how to "switch" data in a UITableView backed by a NSFetchedResultsController, where "switch" means having a different predicate, or even (not in my case) a different entity. Note for the sake of completeness, that since the many-to-many relationship from Selection to Document is ordered, it is handled through an in-between lightweight entity called DocInSelection, which has an "ordering" property and two many-to-one relationships to Document and Selection. Thanks for any suggestion.

Original source