Question

I have a Core Data iPhone app that displays Subscription entities where any of its items are not read. In other words, I construct a predicate like this:

[NSPredicate predicateWithFormat:@"ANY items.read == NO"]

While this works for the initial fetch, it doesn’t affect Subscription entities when I modify an Item, so the NSFetchedResultsController never reevaluates the Subscription entities. What would be a better way of structuring this so that the Subscription entity will be updated whenever an item’s read property is set?

I did try creating a property unreadCount on Subscription and using keyPathsForValuesAffectingUnreadCount to return a set containing items.read. I didn’t expect this to work, and it didn’t. I get an exception from _NSFaultingMutableSet telling me that the read key is not supported.

Answer 1

I see two solutions:

1. When you modify an item have the fetched results controller perform the fetch again and reload the table view.
2. Add a property to Subscription (such as your unreadCount or a boolean hasUnreadItems) and keep it properly updated. Use this property in your fetched results controller.

You can get the set of unread items for a Subscription named “aSubscription” like this:

NSPredicate *pred = [NSPredicate predicateWithFormat:@"read == NO"];
NSSet *unreadItems = [aSubscription.items filteredSetUsingPredicate:pred];