iPhone coredata fetch request, basics around relationships and sections

Question

I've got what is hopefully a simple question - I've got two entities - List and ListItem - and there's a one-to-many relationship set up between them, all good.

My problem comes when I'm trying to perform a fetchrequest which will return the listitems sectioned up by an attribute of the listitem. I can't perform a fetchrequest directly on the listItem objects because there's no way to say 'Where List = 'xxxxx'', so I had something like the following:

- (NSFetchedResultsController *)getListItems {
    // Init a fetch request
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"List" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"listItem.productName" ascending:YES selector:nil];
    NSArray *descriptors = [NSArray arrayWithObject:sortDescriptor];
    [fetchRequest setSortDescriptors:descriptors];


    // Init the fetched results controller
    NSError *error;
    self.globalFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:@"listItem.productName" cacheName:@"listItems"];
    self.globalFetchedResultsController.delegate = self;
    if (![[self globalFetchedResultsController] performFetch:&error])   NSLog(@"Error: %@", [error localizedDescription]);

    [fetchRequest release];
    [sortDescriptor release];

    return self.globalFetchedResultsController;
}

Now, this errors because it's saying I can't use a one-to-many relationship in the sort descriptor - but it also requires that I have a sort descriptor, so I'm not sure the correct way to be doing this. Any help would be great.

Answer 1

If you want to fetch and section ListItems, then you should be using "ListItem" as your entity for the fetch.

In this case, there is a way to say "fetch all ListItem objects Where List = 'xxxxx'", it's called a predicate (NSPredicate).

Try something like this (where "list" is the name of the ListItem relationship with List):

NSPredicate *pred = [NSPredicate predicateWithFormat:@"list == %@", listObject];
[fetchRequest setPredicate:pred];

Also, see the Predicate Programming Guide.