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

AFAIK, you must only use collection operators, described in the Key-Value Coding Programming Guide, for the key paths of your sort descriptors with collections (NSArray, NSSet, NSDictionary).

You must not use collection operators for the key paths of your sort descriptors with fetch requests (NSFetchRequest).

@kdbdallas correctly uses a collection operator to sort an NSArray. He is not using it to specify the sort descriptor of a fetch request.

It'd be sweet though if collection operators worked for specifying the key paths by which to sort fetch requests. Please submit this feature request at http://bugreport.apple.com/. The more it's reported, the more likely they'll support it.

Answer 2

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 3

Since iOS 13 (and friends) you should be able to create a derived attribute called historyMaxTime with the derivation expression max:(history.time) and then set fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"historyMaxTime" ascending:NO]]