How to get objects after CoreData Context merged

Question

Hi,

I tried to save data and merge with CoreData and multi-thread for iPhone app. But I can't get managed objects in the main thread after merging.

I wrote code just like this:

[managedObjectContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:)
                                       withObject:notification
                                    waitUntilDone:YES];

[self performSelectorOnMainThread:@selector(didMerged:) withObject:objectIds waitUntilDone:YES];

So I tried to pass objectIds to get NSManagedObject instances in the main thread which were generated in another thread. At first I tried "objectWithId" method but it generated fault objects. Then I tried "existingObjectWithID" method but it generated objects partly and others were nil with following Error:

[Error] Error Domain=NSCocoaErrorDomain Code=133000 "Operation could not be completed. (Cocoa error 133000.)"

What is wrong? Is there any way how to retrieve all objects by objectIds after merging in another thread?

Thank you.

Answer 1

It seems your context merge failed.

developer documentation on error 133000

NSManagedObjectReferentialIntegrityError = 133000

NSManagedObjectReferentialIntegrityError Error code to denote an attempt to fire a fault pointing to an object that does not exist. The store is accessible, but the object corresponding to the fault cannot be found. Available in Mac OS X v10.4 and later. Declared in CoreDataErrors.h.

Answer 2

First, you need to unroll your errors. Change the output to:

NSLog(@"Error: %@\n%@", [error localizedDescription], [error userInfo]);

That will give you a lot more information.

Second, if you are working with a single context in multiple threads you are doing it wrong. You need to review the documentation on Core Data and threading. The basic rule is: One Context Per Thread; Period. If you need to manage data across multiple threads look into watching the save notifications from the background threads on the main thread. I would suggest reviewing my articles on the Mac Developer Network for examples of this.