NSSortDescriptor ignores Comperator Block

Thanks to the hint from Paul I was able to figure out what the problem is. The solution can be found in the Apple docs(have a look at the "Fetch Predicates and Sort Description" section).

I thought the NSFetchRequest would just use SQL to fetch the Objects from the SQLite DB and then use the SortDescriptions to sort the fetched objects in memory. This is not ture: it seems that the complete sorting work is handed over to the SQLite store as well. Of course the Objective-C statements form the block cannot be translated into a SQL query and thus the block is ignored.

referred to link:

http://stackoverflow.com/questions/11022287/nssortdescriptor-ignores-comperator-block