Exception thrown in NSOrderedSet generated accessors

Question

On my Lion app, I have this data model:

The relationship subitem

Answer 1

This issue occurred to me while migrating a project from Objective-C to Swift 2 with XCode 7. That project used to work, and for a good reason: I was using MOGenerator which had replacement methods to fix this bug. But not all methods require a replacement.

So here's the complete solution with an example class, relying on default accessors as much as possible.

Let's say we have a List with ordered Items

First a quick win if you have a one/to-many relationship, the easiest is to just do:

item.list = list

instead of

list.addItemsObject(item)

Now, if that's not an option, here's what you can do:

// Extension created from your DataModel by selecting it and
// clicking on "Editor > Create NSManagedObject subclass…"

extension List {
  @NSManaged var items: NSOrderedSet?
}

class List

  // Those two methods work out of the box for free, relying on
  // Core Data's KVC accessors, you just have to declare them
  // See release note 17583057 https://developer.apple.com/library/prerelease/tvos/releasenotes/DeveloperTools/RN-Xcode/Chapters/xc7_release_notes.html
  @NSManaged func removeItemsObject(item: Item)
  @NSManaged func removeItems(items: NSOrderedSet)

  // The following two methods usually work too, but not for NSOrderedSet
  // @NSManaged func addItemsObject(item: Item)
  // @NSManaged func addItems(items: NSOrderedSet)

  // So we'll replace them with theses

  // A mutable computed property
  var itemsSet: NSMutableOrderedSet {
    willAccessValueForKey("items")
    let result = mutableOrderedSetValueForKey("items")
    didAccessValueForKey("items")
    return result
  }

  func addItemsObject(value: Item) {
    itemsSet.addObject(value)
  }

  func addItems(value: NSOrderedSet) {
    itemsSet.unionOrderedSet(value)
  }
end

Of course, if you're using Objective-C, you can do the exact same thing since this is where I got the idea in the first place :)