I'm trying to wrap my head around the proper architecture/pattern for this situation:
I've got two entities, Book and Librarys. They each have a many-to-many relationship to the other (Books can be in many Librarys, and a Library will have many Books.
One of my views lists books in a particular library. I have a controller class that handles fetching of this data and provides it to my view. To do that, I'm planning on using an NSFetchRequest for all Book entities, filtered by a predicate that fetches only books in a specific Library. But I can't seem to find the proper way to format the predicate for this fetch request.
I also investigated simply accessing Library's books accessor to get access to the appropriate books without having to fetch anything (as described here), but I want to use NSFetchedResultsControllerDelegate so my controller is notified about any changes to the fetched objects, and can notify the view. I considered just listening for NSManagedObjectContextDidChangeObjectsNotification, but this will deliver notifications for every single change in the context, even if it's not for relevant entities.
How have you handled situations like this in the past?
Your fetch request / predicate could look something like this:
let fetchRequest: NSFetchRequest<Book> = Book.fetchRequest()
fetchRequest.predicate = NSPredicate(format: "%K CONTAINS %@", #keyPath(Book.libraries), library)
// Another option:
fetchRequest.predicate = NSPredicate(format: "SELF IN %@", library.books!)