entity attribute value - Page 347

NSFetchedResultsController fetch request - updating predicate and UITableView

- by Macatomy

In my iPhone Core Data app I have it configured in a master-detail view setup. The master view is a UITableView that lists objects of the List entity. The List entity has a to-many relationship with the Task entity (called "tasks"), and the Task entity has an inverse to-one relationship with List called "list". When a List object is selected in the master view, I want the detail view (another UITableView) to list the Task objects that correspond to that List object. What I've done so far is this: In the detail view controller I've declared a property for a List object: @property (nonatomic, retain) List *list; Then in the master view controller I use this table view delegate method to set the list property of the detail view controller when a list is selected: - (void)tableView:(UITableView *)aTableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath { NSManagedObject *selectedObject = [[self fetchedResultsController] objectAtIndexPath:indexPath]; detailViewController.list = (List*)selectedObject; } Then, I've overriden the setter for the list property in the detail view controller like this: - (void)setList:(List*)newList { if (list != newList) { [list release]; list = [newList retain]; NSPredicate *newPredicate = [NSPredicate predicateWithFormat:@"(list == %@)", list]; [NSFetchedResultsController deleteCacheWithName:@"Root"]; [[[self fetchedResultsController] fetchRequest] setPredicate:newPredicate]; NSError *error = nil; if (![[self fetchedResultsController] performFetch:&error]) { NSLog(@"Unresolved error %@, %@", error, [error userInfo]); abort(); } } } What I'm doing here is setting a predicate on the fetched results to filter out the objects so that I only get the ones that belong to the selected List object. The fetchedResultsController getter for the detail view controller looks like this: - (NSFetchedResultsController *)fetchedResultsController { if (fetchedResultsController == nil) { NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Task" inManagedObjectContext:managedObjectContext]; [fetchRequest setEntity:entity]; NSPredicate *predicate = [NSPredicate predicateWithFormat:@"FALSEPREDICATE"]; [fetchRequest setPredicate:predicate]; NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES]; NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil]; [fetchRequest setSortDescriptors:sortDescriptors]; NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"]; aFetchedResultsController.delegate = self; self.fetchedResultsController = aFetchedResultsController; [aFetchedResultsController release]; [fetchRequest release]; [sortDescriptor release]; [sortDescriptors release]; } return fetchedResultsController; } Its almost unchanged from the default in the Core Data project template, the change I made is to add a predicate that always returns false, the reason being that when there is no List selected I don't want any items to be displayed in the detail view (if a list is selected the predicate is changed in the setter for the list property). However, when I select a list item, nothing really happens. Nothing in the table view changes, it stays empty. I'm sure my logic is flawed in several places, advice is appreciated Thanks

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