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
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.