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PONSSortDescriptor of NSFetchRequest not working after context save
 

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  1. PONSSortDescriptor of NSFetchRequest not working after context save
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    <p>I'm doing operations in a GCD dispatch queue on a NSManagedObjectContext defined like this:</p> <pre><code>- (NSManagedObjectContext *)backgroundContext { if (backgroundContext == nil) { self.backgroundContext = [NSManagedObjectContext MR_contextThatNotifiesDefaultContextOnMainThread]; } return backgroundContext; } </code></pre> <p><code>MR_contextThatNotifiesDefaultContextOnMainThread</code> is a method from <a href="https://github.com/magicalpanda/MagicalRecord" rel="nofollow">MagicalRecord</a>:</p> <pre><code>NSManagedObjectContext *context = [[self alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType]; [context setParentContext:[NSManagedObjectContext MR_defaultContext]]; return context; </code></pre> <p>After fetching my objects and giving them the correct queue position i log them and the order is correct. However, the second log seems to be completely random, the sort descriptor clearly isn't working.</p> <p>I have narrowed down the Problem to <code>[self.backgroundContext save:&amp;error]</code>. After saving the background context sort descriptors are broken.</p> <pre><code>dispatch_group_async(backgroundGroup, backgroundQueue, ^{ // ... for (FooObject *obj in fetchedObjects) { // ... obj.queuePosition = [NSNumber numberWithInteger:newQueuePosition++]; } NSFetchRequest *f = [NSFetchRequest fetchRequestWithEntityName:[FooObject entityName]]; f.predicate = [NSPredicate predicateWithFormat:@"queuePosition &gt; 0"]; f.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"queuePosition" ascending:YES]]; NSArray *queuedObjects = [self.backgroundContext executeFetchRequest:f error:nil]; for (FooObject *obj in queuedObjects) { DLog(@"%@ %@", obj.queuePosition, obj.title); } if ([self.backgroundContext hasChanges]) { DLog(@"Changes"); NSError *error = nil; if ([self.backgroundContext save:&amp;error] == NO) { DLog(@"Error: %@", error); } } queuedObjects = [self.backgroundContext executeFetchRequest:f error:nil]; for (FooObject *obj in queuedObjects) { DLog(@"%@ %@", obj.queuePosition, obj.title); } }); </code></pre> <p>I've got no idea why the sort descriptor isn't working, any Core Data experts want to help out?</p> <p><strong>Update:</strong></p> <p>The problem does not occur on iOS 4. I think the reason is somewhere in the difference between thread isolation and private queue modes. MagicalRecord automatically uses the new concurrency pattern which seems to behave differently.</p> <p><strong>Update 2:</strong></p> <p>The problem has been solved by adding a save of the background context:</p> <pre><code>if ([[NSManagedObjectContext MR_contextForCurrentThread] hasChanges]) { DLog(@"Changes"); NSError *error = nil; if ([[NSManagedObjectContext MR_contextForCurrentThread] save:&amp;error] == NO) { DLog(@"Error: %@", error); } else { NSManagedObjectContext *parent = [NSManagedObjectContext MR_contextForCurrentThread].parentContext; [parent performBlockAndWait:^{ NSError *error = nil; if ([parent save:&amp;error] == NO) { DLog(@"Error saving parent context: %@", error); } }]; } } </code></pre> <p><strong>Update 3:</strong></p> <p>MagicalRecord offers a method to recursively save a context, now my code looks like this:</p> <pre><code>if ([[NSManagedObjectContext MR_contextForCurrentThread] hasChanges]) { DLog(@"Changes"); [[NSManagedObjectContext MR_contextForCurrentThread] MR_saveWithErrorHandler:^(NSError *error) { DLog(@"Error saving context: %@", error); }]; } </code></pre> <p>Shame on me for not using it in the first place...</p> <p>However, I don't know <em>why</em> this helps and would love to get an explanation.</p>
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