How can NSArrayController sort on a to-many relationship?

Question

Using CoreData, I have an entity "Bookmark", that has an to-many relationship named 'tags' to another entity "Tag", and some commun attributes (string, date, ...).

In a NSTableView we display the Bookmarks entity via Binding:

• the NSArrayController is binded to File's Owner.managedObjectContext (standard XCode CoreData template, the managedObjectContext is in the AppDelegate)
• The columns in TableView are binded to their respective attribute. In particular the Tag column is binded to this arrayController.arrangedObjects.tags with a subclass of NSValueTransformer so that we can show, as an NSString, a summary of the to-many relationship.

It work. Now when I click on column header the whole table view get sorted correctly except for the 'tag' column where I get this:

-[_NSFaultingMutableSet compare:]: unrecognized selector sent to instance

For sure the "Set" from this to-many relationship doesn't respond to the selector 'compare:'.

Question: How can I make this work ? How can I sort on a to-many relationship ?

Are something like the ValueTransformer available ? If I could supply a custom class that would do the compare: for the ArrayController to know...

Answer 1

One possible hack: since _NSFaultingMutableSet is a NSSet, we can add the selector 'compare:' via a categorie.

@interface NSSet (someAdditions)
   - (NSComparisonResult)compare:(NSSet *)anotherSet;
@end

@implementation NSSet (someAdditions)
   - (NSComparisonResult)compare:(NSSet *)aSet {
      ...  
   }
@end

we can now implement this compare: selector as we wish, like comparing the count of each set, or their NSString representation in some way.

It work in my App. I re-enabled the 'Creates Sort Descriptor' on the binding of the NSTableColumn and can now click on the header of my tableView to sort.

It's a hack because it affect all NSSet... But at least I have my hook.

What do you think ?