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PO"Collection was mutated while being enumerated" on executeFetchRequest
 

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  1. PO"Collection was mutated while being enumerated" on executeFetchRequest
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    <p>I'm stuck on a problem for hours now and having read everything about this on stackoverflow (and apply every advices found), I'm now officially in need for help. ;o)</p> <p>Here is the context :</p> <p>In my iPhone project, I need to import data on the background and insert it in a managed object context. Following the advices found here, here is what I'm doing :</p> <ul> <li>Save the main moc</li> <li>Instantiate a background moc with the persistent store coordinator used by the main moc</li> <li>Register my controller as an observer of the NSManagedObjectContextDidSaveNotification notification for the background moc</li> <li>Call the import method on a background thread</li> <li>Each time data is received, insert it on the background moc</li> <li>Once all the data has been imported, save the background moc</li> <li>Merge the changes into the main moc, on the main thread</li> <li>Unregister my controller as an observer for the notification</li> <li>Reset and release the background moc</li> </ul> <p>Sometimes (and randomly), the exception...</p> <pre><code>*** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection &lt;__NSCFSet: 0x5e0b930&gt; was mutated while being enumerated... </code></pre> <p>...is thrown when I call executeFetchRequest on the background moc, to check if the imported data already exists in the database. I wonder what is mutating the set since there is nothing that run outside the import method.</p> <p>I've included the entire code of my controller and my test entity (my project consisting of these two classes and the app delegate, which has been unmodified) :</p> <pre><code>// // RootViewController.h // FK1 // // Created by Eric on 09/08/10. // Copyright (c) 2010 __MyCompanyName__. All rights reserved. // #import &lt;CoreData/CoreData.h&gt; @interface RootViewController : UITableViewController &lt;NSFetchedResultsControllerDelegate&gt; { NSManagedObjectContext *managedObjectContext; NSManagedObjectContext *backgroundMOC; } @property (nonatomic, retain) NSManagedObjectContext *managedObjectContext; @property (nonatomic, retain) NSManagedObjectContext *backgroundMOC; @end // // RootViewController.m // FK1 // // Created by Eric on 09/08/10. // Copyright (c) 2010 __MyCompanyName__. All rights reserved. // #import "RootViewController.h" #import "FK1Message.h" @implementation RootViewController @synthesize managedObjectContext; @synthesize backgroundMOC; - (void)viewDidLoad { [super viewDidLoad]; self.navigationController.toolbarHidden = NO; UIBarButtonItem *refreshButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemRefresh target:self action:@selector(refreshAction:)]; self.toolbarItems = [NSArray arrayWithObject:refreshButton]; } #pragma mark - #pragma mark ACTIONS - (void)refreshAction:(id)sender { // If there already is an import running, we do nothing if (self.backgroundMOC != nil) { return; } // We save the main moc NSError *error = nil; if (![self.managedObjectContext save:&amp;error]) { NSLog(@"error = %@", error); abort(); } // We instantiate the background moc self.backgroundMOC = [[[NSManagedObjectContext alloc] init] autorelease]; [self.backgroundMOC setPersistentStoreCoordinator:[self.managedObjectContext persistentStoreCoordinator]]; // We call the fetch method in the background thread [self performSelectorInBackground:@selector(_importData) withObject:nil]; } - (void)_importData { NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(backgroundMOCDidSave:) name:NSManagedObjectContextDidSaveNotification object:self.backgroundMOC]; FK1Message *message = nil; NSFetchRequest *fetchRequest = nil; NSEntityDescription *entity = [NSEntityDescription entityForName:@"FK1Message" inManagedObjectContext:self.backgroundMOC]; NSPredicate *predicate = nil; NSArray *results = nil; // fake import to keep this sample simple for (NSInteger index = 0; index &lt; 20; index++) { predicate = [NSPredicate predicateWithFormat:@"msgId == %@", [NSString stringWithFormat:@"%d", index]]; fetchRequest = [[[NSFetchRequest alloc] init] autorelease]; [fetchRequest setEntity:entity]; [fetchRequest setPredicate:predicate]; // The following line sometimes randomly throw the exception : // *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection &lt;__NSCFSet: 0x5b71a00&gt; was mutated while being enumerated. results = [self.backgroundMOC executeFetchRequest:fetchRequest error:NULL]; // If the message already exist, we retrieve it from the database // If it doesn't, we insert a new message in the database if ([results count] &gt; 0) { message = [results objectAtIndex:0]; } else { message = [NSEntityDescription insertNewObjectForEntityForName:@"FK1Message" inManagedObjectContext:self.backgroundMOC]; message.msgId = [NSString stringWithFormat:@"%d", index]; } // We update the message message.updateDate = [NSDate date]; } // We save the background moc which trigger the backgroundMOCDidSave: method [self.backgroundMOC save:NULL]; [[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:self.backgroundMOC]; [self.backgroundMOC reset]; self.backgroundMOC = nil; [pool drain]; } - (void)backgroundMOCDidSave:(NSNotification*)notification { if (![NSThread isMainThread]) { [self performSelectorOnMainThread:@selector(backgroundMOCDidSave:) withObject:notification waitUntilDone:YES]; return; } // We merge the background moc changes in the main moc [self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification]; } @end // // FK1Message.h // FK1 // // Created by Eric on 09/08/10. // Copyright 2010 __MyCompanyName__. All rights reserved. // #import &lt;CoreData/CoreData.h&gt; @interface FK1Message : NSManagedObject { } @property (nonatomic, retain) NSString * msgId; @property (nonatomic, retain) NSDate * updateDate; @end // // FK1Message.m // FK1 // // Created by Eric on 09/08/10. // Copyright 2010 __MyCompanyName__. All rights reserved. // #import "FK1Message.h" @implementation FK1Message #pragma mark - #pragma mark PROPERTIES @dynamic msgId; @dynamic updateDate; @end </code></pre> <p>This is all ! The whole project is here. No table view, no NSFetchedResultsController, nothing else than a background thread that import data on a background moc.</p> <p>What could mutate the set in this case ?</p> <p>I'm pretty sure I'm missing something obvious and it's driving me mad.</p> <p>EDIT: </p> <p>Here is the full stack trace :</p> <pre><code> 2010-08-10 10:29:11.258 FK1[51419:1b6b] *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection &lt;__NSCFSet: 0x5d075b0&gt; was mutated while being enumerated.&lt;CFBasicHash 0x5d075b0 [0x25c6380]&gt;{type = mutable set, count = 0, entries =&gt; } ' *** Call stack at first throw: ( 0 CoreFoundation 0x0255d919 __exceptionPreprocess + 185 1 libobjc.A.dylib 0x026ab5de objc_exception_throw + 47 2 CoreFoundation 0x0255d3d9 __NSFastEnumerationMutationHandler + 377 3 CoreData 0x02287702 -[NSManagedObjectContext executeFetchRequest:error:] + 4706 4 FK1 0x00002b1b -[RootViewController _fetchData] + 593 5 Foundation 0x01d662a8 -[NSThread main] + 81 6 Foundation 0x01d66234 __NSThread__main__ + 1387 7 libSystem.B.dylib 0x9587681d _pthread_start + 345 8 libSystem.B.dylib 0x958766a2 thread_start + 34 ) terminate called after throwing an instance of 'NSException' </code></pre>
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    1. COIn Xcode's Run menu, turn on “Stop on Objective-C Exceptions”, then run your app under the Debugger. What do you find?
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      1. PO"Collection was mutated while being enumerated" on executeFetchRequest
      1. USPeter Hosey
    2. COIt confirms that the app crash on the "executeFetchRequest:error:" line. I have added the full stack trace to my original question...
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    3. COAnd what about the other threads?
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