How to count list size of parent entity in NSFetchedResultsController

I'm using NSFetchedResultsController and I have a problem to query count in parent attribute.

Assuming that following data model, 'Group', 'Category', 'Item'.

- Item : All items belong to ‘Category’

- Category : ‘Category’ may belong to a certain ‘Gro开发者_运维问答up’

- Group : 'Group' has zero to N 'Category'

And I want to search all items which does not have any groups in category.

My codes are following :

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Item" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
...
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"((category.groups.@count == 0) || category.categoryId == %@)", categoryId];
...

But, "category.groups.@count" does not work in here. (It works well out of NSFetchedRequest)

How can I solve this problem. Please help me;;

Thanks.

A relationship can also be nil.

(((category.groups.@count == 0) || category.groups == nil) || category.categoryId == %@)

The resolution of the code inside of the predicate when you are running against a SQLite backend can be different than what you expect inside of Objective-C directly. When you run into odd things like this it can be helpful to turn on sql debugging to see what the underlying sql is and adjust your predicates accordingly.

You open 2 brackets, and close only one:

...@"((category.groups.@count == 0) || category.categoryId == %@"...