Fetch the smallest date of an object in CoreData - objective-c

Question

I'm having the same problem as this question, but the answer of #davedelong is not working for me.
When following the Apple Example, for fetching the smallest date in a set of object I get the following error
-[NSDate count]: unrecognized selector sent to instance
My understanding is that NSExpression's max: only support NSArrays. So I need an other solution.
#davedelong suggested using an ascending NSSortDescriptor, and so I did :
NSFetchRequest* fetchRequest = [[NSFetchRequest alloc] init];
fetchRequest.entity = [NSEntityDescription entityForName:NSStringFromClass([GCSession class])
inManagedObjectContext:self.objectContext];
fetchRequest.fetchLimit = 1;
NSSortDescriptor* sort = [[NSSortDescriptor alloc] initWithKey:#"startDate" ascending:YES];
fetchRequest.sortDescriptors = [NSArray arrayWithObject:sort];
GCSession* session = [[self.objectContext executeFetchRequest:fetchRequest error:nil] lastObject];
return session.startDate;
The problem here is that the session object returned from the fetch doesn't seems to be the one with the smallest startDate. In my tests, it seems that it even returned the newest date but it doesn't seems consistent.
I could also fetch every GCSession object and sort them but that seems way overkill, especially that GCSession will be augmenting in number when the users will use the application.
Edit : A test project that demonstrate the bug in Apple's example code.

Answer 1

A sort only comes into play after you've fetch the objects. You've set a fetch limit of one and no predicates which tells the fetch "go grab any random single GCSession object". One the fetch has an array 1 element long, it then sorts it, which is useless.
If you want to use a sort to find a min or max, you have to fetch all the objects and then sort them. Removing the line:
fetchRequest.fetchLimit = 1;
… should allow the code to work.
However, you should be able to fetch min and max values with expressions. It's kind of a basic operation.
Edit: Look at the comments for more detail, but Apple's example can work if you change the backing store to SQL instead of XML