My understanding on the background of using this (2 different versions which varies on return values) method is: Managed-Objects created in memory by a MOC are first assigned temporary object IDs so that they may be uniquely identified without involving the persistent store. When a MOC is saved, however, the persistent store coordinator needs permanent object IDs for these Managed-objects. (correct me if I got wrong, please!)
I never used MOC's
- (BOOL)obtainPermanentIDsForObjects:(NSArray *)objects error:(NSError **)error
, but still call MOC's save: method without problems. However the code I used this week by 3rd party, uses this MOC's obtainPermanentIDsForObjects each time right after inserting a new Managed-object, is this really necessary?? Maybe needed for multithreading environment? But why Apple "CoreData PG" never mentioned this method? Has persistent-store-coordinator done it automatically for me just before saving MOC?
During research I saw another version of this method in NSIncrementalStore Class...
- (NSArray *)obtainPermanentIDsForObjects:(NSArray *)array error:(NSError **)error
, it says "This method is called (overridden in subclass then called, I guess) before executeRequest:withContext:error: with a save request, to assign permanent IDs to newly-inserted objects."
What should I do? Do I need to use both versions (NSIncrementalStore subclass's before save, and MOC's version right after new creation)? Which method is must called OR neither?
Thanks!
When an NSManagedObject is first inserted into an NSManagedObjectContext it has a temporary NSManagedObjectID. When the context is saved, the NSManagedObjectContext will do the equivalent of obtainPermanentIDsForObjects:error on NSManagedObjectContext, which in turn calls NSPersistentStoreCoordinator, which finds the store responsible for this object and calls obtainPermanentIDsForObjects:error on the store itself. This turns the NSManagedObject's temporary ID into a permanent one.
NSManagedObjectIDs can be passed between NSManagedObjectContexts (if they share the same stores), but only if the NSManagedObjectID is a permanent ID. You can call obtainPermanentIDsForObjects:error to turn a temporary ID into a permanent ID independent of a save operation, but the store that the object belongs to may not expect that.