CoreData could not fulfill a fault for

Question

I have a really annoying problem, which I just can't seem to get fixed.

I have a view when I send a message that gets saved to the Core Data, when thats done it asked the database for a random message (sentence) and saved that as well to an other row in the database.

If I do the last part hardcoded, without fetching data from the DB, it works all fine and dandy, but as soon as I fetch the random row from the DB it goes crazy.

In my AppDelegate.m:

- (void)save {
    NSAssert(self.context != nil, @"Not initialized");
    NSError *error = nil;
    BOOL failed = [self.context hasChanges] && ![self.context save:&error];
    NSAssert1(!failed,@"Save failed %@",[error userInfo]);
}

- (NSString*)selectRandomSentence
{
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Sentences" inManagedObjectContext:self.managedObjectContext];
    [request setEntity:entity];

    NSError *error = nil;
    NSUInteger count = [self.context countForFetchRequest:request error:&error];

    NSUInteger offset = count - (arc4random() % count);
    [request setFetchOffset:offset];
    [request setFetchLimit:1];

    NSArray *sentenceArray = [self.context executeFetchRequest:request error:&error];

    [request release];

    return [[sentenceArray objectAtIndex:0] sentence];
}

- (NSManagedObjectContext *)context {

    if (_managedObjectContext != nil)
        return _managedObjectContext;

    NSPersistentStoreCoordinator *coordinator = [self coordinator];
    if (coordinator != nil) {
        _managedObjectContext = [[NSManagedObjectContext alloc] init];
        [_managedObjectContext setPersistentStoreCoordinator:coordinator];
    }

    return _managedObjectContext;
}

In my ChatController.m:

- (void)didRecieveMessage:(NSString *)message
{
    [self addMessage:message fromMe:NO];
}

#pragma mark -
#pragma mark SendControllerDelegate

- (void)didSendMessage:(NSString*)text {
    [self addMessage:text fromMe:YES];
}

#pragma mark -
#pragma mark Private methods

- (void)responseReceived:(NSString*)response {
    [self addMessage:response fromMe:NO];
}

- (void)addMessage:(NSString*)text fromMe:(BOOL)fromMe {
    NSAssert(self.repository != nil, @"Not initialized");
    Message *msg = [self.repository messageForBuddy:self.buddy];
    msg.text = text;
    msg.fromMe = fromMe;

    if (fromMe)
    {
        [self.bot talkWithBot:text];
    }

    [self.repository asyncSave];

    [self.tableView reloadData];
    [self.tableView scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:[self.buddy.messages count] - 1] atScrollPosition:UITableViewScrollPositionBottom animated:YES];
}

In My OfflineBot.m:

- (void)talkWithBot:(NSString *)textFromMe
{
    AppDelegate *delegate = [[UIApplication sharedApplication] delegate];
    [self didRecieveMessage:[delegate selectRandomSentence]];
}

- (void)didRecieveMessage:(NSString *)message
{
    if ([self.delegate respondsToSelector:@selector(didRecieveMessage:)])
        [self.delegate didRecieveMessage:message];
}

Repository.m

- (Message*)messageForBuddy:(Buddy*)buddy {
    Message *msg = [self.delegate entityForName:@"Message"];
    msg.source = buddy;
    [self.delegate.managedObjectContext refreshObject:buddy mergeChanges:YES];
    return msg;
}

- (void)asyncSave {
    [self.delegate save];
}

The error:

2012-08-10 00:28:20.526 Chat[13170:c07] * Assertion failure in -[AppDelegate save], /Users/paulp/Desktop/TestTask/Classes/AppDelegate.m:28 2012-08-10 00:28:20.527 Chat[13170:c07] * Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Save failed {type = immutable dict, count = 2, entries => 1 : {contents = "NSAffectedObjectsErrorKey"} = ( " (entity: Sentences; id: 0x6b8bf10 ; data: )" ) 2 : {contents = "NSUnderlyingException"} = CoreData could not fulfill a fault for '0x6b8bf10 ' }

What am I doing wrong?

Update I pinpointed the error to this row:

NSArray *sentenceArray = [self.context executeFetchRequest:request error:&error];

When I execute that row, I get the error... that is when fetching the data. The error, however, seems to turn up when saving the new data to the Messages entity. The random sentence is fetched from Sentences.

After I changed the asyncSave method to saving directly (thus not using a new thread) it saves the first chat, but nothing after that. It dies.

Update It all seems to work using this in my didFinishLaunchingWithOptions:

[self.context setRetainsRegisteredObjects:YES];

I understand that hereby the CodeData Object Model Context doesn't release its objects, which seems to be the problem betweens adding and saving. But why?

Answer 1

Hmm. Are you properly implementing concurrency following this guide? The problem you are seeing is a common one when using core data across multiple threads. Object was deleted in your "background context", while then it's being accessed by another context. Calling [context processPendingChanges] on your background context after the delete but before the save may help.

There is also a WWDC 2010 session (137) on optimizing core data performance that goes into deletes a bit.

When you execute a fetch Core Data returns a collection of objects matching the predicate you supplied. Those objects don't actually have their property values set yet. It's when you access a property that Core Data goes back to the store to "fire the fault" - populate the property with data from the store. "Could not fulfill a fault..." exceptions happen when Core Data goes to the store to get the property values for an object, but the object does not exist in the persistent store. The managed object context thought it should exist, which is why it could attempt the fault - which is where the problem is. The context that caused the exception to be thrown did not know that this object had been deleted from the store by something else (like another context).

Note the the above Concurrency Guide is now out of date, you should be using parent-child contexts and private queue concurrency rather than the older thread confinement model. Parent-child contexts are much less likely to run into "Could not fulfill a fault..." for many reasons. And please, file a documentation bug or use the feedback form to request that the concurrency guide be updated.