views:

169

answers:

1

Background

I've got the following tree of objects:

Name                       Project       
Users                      nil           
  John                     nil            
    Documents              nil           
      Acme Project         Acme Project    <--- User selects a project
        Proposal.doc       Acme Project  
          12:32-12:33      Acme Project  
          13:11-13:33      Acme Project  
            ...thousands more entries here...
  • The user can assign a group to a project. All descendants get set to that project.

  • This locks up the main thread so I'm using NSOperations.

  • I'm using the Apple approved way of doing this, watching for NSManagedObjectContextDidSaveNotification and merging into the main context.

The Problem

My saves have been failing with the following error:

Failed to process pending changes before save. The context is still dirty after 100 attempts. Typically this recursive dirtying is caused by a bad validation method, -willSave, or notification handler.

What I've Tried

I've stripped all the complexities of my app away, and made the simplest project I could think of. And the error still occurs. I've tried:

  • Setting the max number of operations on the queue to 1 or 10.

  • Calling refreshObject:mergeChanges: at several points in the NSOperation subclass.

  • Setting merge policies on the managed object context.

  • Build and Analyze. It comes up empty.

My Question

How do I set relationships in an NSOperation without my app crashing? Surely this can't be a limitation of Core Data? Can it?

The Code

Download my project: http://synapticmishap.co.uk/CDMTTest1.zip

Main Controller

@implementation JGMainController

-(IBAction)startTest:(id)sender {
    NSManagedObjectContext *imoc = [[NSApp delegate] managedObjectContext];

    JGProject *newProject = [JGProject insertInManagedObjectContext:imoc];
    [newProject setProjectName:@"Project"];
    [imoc save];

        // Make an Operation Queue
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [queue setMaxConcurrentOperationCount:1]; // Also crashes with a higher number here (unsurprisingly)

    NSSet *allTrainingGroupsSet = [imoc fetchAllObjectsForEntityName:@"TrainingGroup"];

    for(JGTrainingGroup *thisTrainingGroup in allTrainingGroupsSet) {
        JGMakeRelationship *makeRelationshipOperation = [[JGMakeRelationship alloc] trainGroup:[thisTrainingGroup objectID] withProject:[newProject objectID]];
        [queue addOperation:makeRelationshipOperation];
        makeRelationshipOperation = nil;
    }
}

    // Called on app launch.
-(void)setupLotsOfTestData {
         // Sets up 10000 groups and one project
}

@end

Make Relationship Operation

@implementation JGMakeRelationshipOperation

-(id)trainGroup:(NSManagedObjectID *)groupObjectID_ withProject:(NSManagedObjectID *)projectObjectID_ {
    appDelegate = [NSApp delegate];
    imoc = [[NSManagedObjectContext alloc] init];
    [imoc setPersistentStoreCoordinator:[appDelegate persistentStoreCoordinator]];
    [imoc setUndoManager:nil];
    [imoc setMergePolicy:NSMergeByPropertyStoreTrumpMergePolicy];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(mergeChanges:) 
                                                 name:NSManagedObjectContextDidSaveNotification 
                                               object:imoc];
    groupObjectID = groupObjectID_;
    projectObjectID = projectObjectID_;
    return self;
}

-(void)main {
    JGProject       *project        = (JGProject *)[imoc objectWithID:projectObjectID];
    JGTrainingGroup *trainingGroup = (JGTrainingGroup *)[imoc objectWithID:groupObjectID];
    [project addGroupsAssignedObject:trainingGroup];
    [imoc save];

    trainingGroupObjectIDs = nil;
    projectObjectID = nil;
    project = nil;
    trainingGroup = nil;
}

-(void)mergeChanges:(NSNotification *)notification {
    NSManagedObjectContext *mainContext = [appDelegate managedObjectContext];
    [mainContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:)
                                  withObject:notification
                               waitUntilDone:YES];  
}

-(void)finalize {
    appDelegate = nil;
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    imoc = nil;
    [super finalize];
}
@end


@implementation NSManagedObjectContext (JGUtilities)

-(BOOL)save {
         // If there's an save error, I throw an exception
}

@end

Data Model

Data Model

Update 1

I've experimented some more, and even without the merge, the exception is still thrown. Just saving the managed object context in another thread after modifying a relationship is enough.

I have a shared persistent store coordinator with the app delegate. I've tried making a separate NSPersistentStoreCoordinator for the thread with the same URL as my data store, but Core Data complains.

I'd love to suggestions on how I can make a coordinator for the thread. The core data docs allude to there being a way of doing it, but I can't see how.

+3  A: 

You are crossing the streams (threads in this case) which is very bad in CoreData. Look at it this way:

  1. startTest called from a button (is IBAction, assuming button tap) on Main thread
  2. Your for loop creates a JGMakeRelationship object using the initializer trainGroup: withProject: (this should be called init, and probably call super, but that's not causing this issue).
  3. You create a new managed object context in the operation, on the Main thread.
  4. Now the operation queue calls the operations "main" method from a worker thread (put a breakpoint here and you'll see it's not on the main thread).
  5. Your app goes boom because you've accessed a Managed object Context from a different thread than the one you created it on.

Solution:

Initialize the managed object context in the main method of the operation.

ImHuntingWabbits
HOLY CRAP. I cannot believe I didn't see that. You are, sir, a wonderful, wonderful human being. I'll try not to get too excited until I see this working, but I'm hopeful :)
John Gallagher
It's fixed. I can't get over how I just didn't see that. I've been stressing about this for days now. I'm so grateful to you for answering this question. Please hit me up on twitter (synapticmishap) and I'll give you a copy of my app/some other reward.
John Gallagher
LOL glad it worked out :) Try not to be too hard on yourself, I've made that mistake quite a few times.
ImHuntingWabbits