NSSortDescriptor and to-many relationships

Question

I have two types of objections: locations and history items. I'm trying to fetch locations which are attached to any history item, so my fetch predicate for the location is "history.@count > 0", which works fine.

I'd also like to sort the location objects with an NSSortDescriptor by the date of their latest history item, which as far as I can make out would be "[email protected]", this however throws the following error:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException',
    reason: 'Keypath containing KVC aggregate where there shouldn't be one;
    failed to handle [email protected]'

Halp plox?

Answer 1

Probably not the best solution, but you could always pull back the data and sort it after the fact. Have a look at Sorting and Filtering NSArray Objects.

Answer 2

Here is some code that I have where I do similar

NSError *error;

if (![[self fetchedResultsController] performFetch:&error])
{
 UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:@"Error" message:[error localizedDescription] delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil] autorelease];
 [alert show];
}
else
{
 self.feedsArray = [fetchedResultsController fetchedObjects];

 NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"[email protected]" ascending:NO];
 NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

 self.feedsArray = [feedsArray sortedArrayUsingDescriptors:sortDescriptors];

 [sortDescriptors release];
 [sortDescriptor release];

 NSInteger overviewAmount = [[feedsArray valueForKeyPath:@"@sum.unreadArticles"] integerValue];

    ....
}