How can I sort by a transformable attribute in an NSFetchedResultsController?

Question

I'm using NSValueTransformers to encrypt attributes (strings, dates, etc.) in my Core Data model, but I'm pretty sure it's interfering with the sorting in my NSFetchedResultsController.

Does anyone know if there's a way to get around this? I suppose it depends on how the sort is performed; if it's always only performed directly on the database, then I'm probably out of luck. If it sorts on the objects themselves, then perhaps there's a way to activate the transformation before the sort occurs.

I'm guessing it's directly on the database, though, since the sort would be key in grabbing subsets of the collection, which is the main benefit of NSFetchedResultsController anyway.

Note: I should add that there's some strange behavior here... the collection doesn't sort in the first session (the session where the objects are created), but it does sort in subsequent sessions (where the objects already exist and are just being retrieved). So perhaps sorting does work with transformables, but maybe there is caveat in that they have to be saved first or something like that (?)

Answer 1

If you are sorting within the NSFetchedResultsController then it is against the store (i.e. database). However, you can perform a "secondary" sort against the results when they are in memory and therefore decrypted by calling -sortedArrayUsingDescriptors:

update

I believe your inconsistent behavior is probably based on what is already in memory vs. what is being read directly from disk.