Sort NSFetchedResultsController result based on array?

Question

I would like to construct a NSPredicate or NSSortDescriptor (Core Data search) that is based on the content of an array.

The array will consists of userId:s in the right order:

[1, 2, 5, 3];

I would like to present the result for my NSFetchedResultsController in the same order.

So I will:

1. Get the right users (I have done this)
2. Sort the list of users based on my sorted array with userIds.

Is this possible to do, and how can I do it?

Answer 1

I think that is not possible. Sorting the results of a fetch request (for a SQLite-based store) is done on the SQLite level, and can use only persistent attributes stored in the database. The sort descriptor cannot use Objective-C functions or variables.

So either

• you add an additional attribute to your entity that describes the intended ordering (if that is possible), or
• you sort the results according to your needs after fetching them (which means that you cannot use a fetched results controller for automatic change tracking anymore). Perhaps you can use the method described here: https://stackoverflow.com/a/15618277/1187415.