NSSortDescriptor sort by max property value in NSSet

Question

I am trying use NSSortDescriptor and sorts "Units" which have a set of "Devices". Each device has a signal strength. I would like to sort units by the device with the highest signal strength. I just found a page about collection operators and I think I'm close but I still can't quite get it to work.

NSSortDescriptor *rssiSortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"[email protected]" ascending:[self sortByRSSIAscending]];    

I'm guessing the issue here is that devices can't be directly compared to each other, so the @max operator doesn't work. Is there a way to override this to compare by signal strength?

I have added this to my device model:

- (NSComparisonResult)compare:(PersistedNokeDevice*)other
{
if (self.signalStrength < other.signalStrength) {
    return NSOrderedAscending;
}
if (self.signalStrength == other.signalStrength) {
    return NSOrderedSame;
}
return NSOrderedDescending;
}

I am getting this error Keypath containing KVC aggregate where there shouldn't be one; failed to handle [email protected]

Is there a way to make this work or do I need to rethink my strategy?