Why is an NSArrayController "losing" its sort after an object is inserted?

I have an NSArrayController bound to a property of an NSManagedObject subclass. The subclass is automatically generated by mogenerator (which creates a set property for the relationship).

The NSArrayController is bound to network.posts where 'posts' is a 1-M relationship. Note I am binding to 'posts' rather than the 'postsSet' mutableSet accessor generated by mogenerator.

The set represents a relationship with another entity. The array controller has a sort descriptor set and, when the window it is associated with 开发者_运维问答opens, data is displayed in the correct sort order.

Then I add a new object by instantiating an entity and then adding it to the relationship.

The NSArrayController is correctly observing this change and the new object appears in arrangedObjects however, after the insert the sort order of arrangedObjects is lost and the records appear in a different order.

I have verified that the sort descriptor is still set correctly. The NSArrayController has autoRearrangeContent=YES. I've even tried manually calling -rearrangeObjects after the insert but the sort order remains wrong.

If I close the window and re-open the newly instantiated NSArrayController has the data with the correct sort order again. Until I do another insert.

My experience has been that NSArrayController has automatically kept the correct sort order when objects are added/removed but maybe that was a lucky coincidence?

I can't find any description of the correct behaviour in the Apple document so I've no idea what to expect, what might be going wrong (if anything), or - in either case - what I should do about it.

1) Given a sortDescriptor should NSArrayController be keeping arrangedObjects sorted after objects are inserted/removed from the underlying collection?

2) If so, what might prevent this from happening?

3) And, if not, what is the correct way to keep arrangedObjects sorted?

I'd be grateful for any help. It's not easy to provide useful source code in this situation since, in principle, there isn't any. But I'm happy to clarify and answer any follow-up questions.

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Turn off lazy fetching. Not sure the reason but when the controller is being lazy it doesn't resort things when they are edited or added.

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By what are you sorting? Core data does not support ordered collections. To preserve any kind of order, you have to add some sort of attribute (pun intended) and sort by it.

Behind the scenes, CD is using NSSet/NSMutableSet - not NSArray/NSMutableArray - for collections. That your objects come up in the same order more than once is entirely due to caching, not by CD maintaining your collection's order.

Update for Lion (10.7)

With regard to my "does not support ordered collections" statement: If you're targeting 10.7 and above in your application, [NSManagedObject now gives you ordered relationships.][1] Use -mutableOrderedSetValueForKey: and -mutableOrderedSetValueForKey: to set and retrieve NSOrderedSets. Yay!

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Perhaps instead of binding the NSArrayController to your attributesSet (generated by mogenerator), you should instead bind to the underlying dynamic attributes property itself, which is created by Core Data framework? attributes stands for your relationship name.