iPhone coredata fetch request, basics around relationships and sections
core-data, iphone, objective-c, relationships
Solution
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.
Problem
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.