Binding an Ordered Relationship with an NSArrayCon

问题:

How does one bind an NSArrayController's content to the entities in an ordered to-many relationship?

I have an unordered to-many relationship in my Core Data model, and an NSArrayController whose Content Set is bound to that relationship from the parent entity. This works fine, the data is accessible from the NSArrayController, no problem.

I decided during development that it would be better to allow users to manually reorder these child objects, so I changed the relationship to an ordered one. Now, when my NSArrayController is being created from my nib, the following error is presented:

Cannot create NSSet from object Relationship '...' fault on managed object ... of class _NSFaultingMutableOrderedSet

Now actually, I think this all makes sense: It's an ordered relationship, so now I'm getting an ordered set. Binding it to Content Array also would be inappropriate, since it's now an NSOrderedSet, not an array. My question is: Now how do I bind this relationship's data back into the NSArrayController?

回答1:

The fundamental problem is that a Core Data ordered to-many relationship returns an NSOrderedSet, and NSOrderedSet is not a subclass of NSSet. Any array controller bindings that expect an NSSet will fail.

Tom Fewster has a detailed blog post describing the use of NSValueTransformer to work around this shortcoming, converting between NSOrderedSet and NSArray on the fly. He also provides a sample implementation on Github.

回答2:

I came across this discussion while searching to see if there've been any new developments on this front. In a shipping app I currently bind the array controller's content array to orderedSetKey.@array and it works just fine, not sure if I discovered that myself or if someone else suggested it somewhere.