Sorting NSSet throws exception valueForUndefinedKey

Question

I'm trying to sort an NSSet of NSManagedObjects. The value for one of the set item's "pubDate" property is nil. The following code throws an exception [NSManagedObject valueForUndefinedKey] when it is sorted. How can I get it to ignore the element without a value for that key?

NSSortDescriptor *sortNameDescriptor = [[[NSSortDescriptor alloc] initWithKey:@"pubDate" ascending:NO] autorelease];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortNameDescriptor, nil];
return [[self.items allObjects] sortedArrayUsingDescriptors:sortDescriptors];

Answer 1

You can't sort objects on a value that doesn't exist. Can you give it a default value? It's like trying to sort the array {1, 5, 3, X, 9, 7}, you just can't unless you tell it how to deal with X.