How to Create a NSSortDescriptor that Sorts by Multiple Keys with One Selector

Question

My data model is as such:

User (Entity)
- valueOne (NSNumber Attribute)
- valueTwo (NSNumber Attribute)

I am using an NSFetchedResultsController to fetch User entities from Core Data to display in my UITableView. I am having trouble sorting the results of the fetch request. I need to sort by valueOne (divided by) valueTwo. I have tried passing both @"self" and nil for the Key value of sortDescriptorWithKey:ascending:selector: to obtain the User object. I would then sort by the User.valueOne / User.valueTwo values in my custom selector, but neither @"self" or nil seem to work, as I get errors for both. The documentation doesn't seem to contain anything about how to pass the parent object for the key to sort by.

How would I go about creating an NSSortDescriptor which sorts by valueOne / valueTwo?

Answer 1

That is not possible. The sort descriptor for a Core Data fetch request can only sort according to (persistent) attributes of the entities. Transient attributes or custom (Objective-C based) comparator methods cannot be used.