IOS - CoreData error causing null values

Question

When I first fill out the view, I do this:

self.cats = [[DataManager sharedManager] getCatsFromCoreData]; //cats is an NSArray property (strong) for (Cat* cat in self.cats) { CatThumbnailView *thumb = [CatThumbnailView catThumbnailView]; thumb.cat = cat; UITapGestureRecognizer * tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(thumbnailTapped:)]; [thumb addGestureRecognizer:tap]; [thumb setText:[cat.name uppercaseString]]; ... [self.someScrollview addSubview:thumb]; yPos += thumb.frame.size.height + spacing; } 

The getManager DataManager method is as follows:

 - (NSArray*)getCatsFromCoreData { NSManagedObjectContext *context = [[CoreDataController sharedController] managedObjectContext]; NSFetchRequest *request = [[NSFetchRequest alloc] init]; [request setEntity:[NSEntityDescription entityForName:@"Cats" inManagedObjectContext:context]]; [request setSortDescriptors:[NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"order" ascending:YES]]]; NSError *error = nil; NSArray *fetchedCats = [context executeFetchRequest:request error:&error]; if (fetchedCats.count > 0) { return fetchedCats; } return nil; } 

Everything works perfectly. The view is populated. The thumbnail click method is as follows:

 - (void)thumbnailTapped:(UITapGestureRecognizer*)tap { CatThumbnailView* thumb = (CatThumbnailView*)tap.view; DLog(@"cat %@", thumb.cat); DLog(@"cat id: %@", thumb.cat.cat_id); //do stuff here with the cat data } 

The problem is that sometimes, when I click on the cat thumbnail, I get the following:

 cat <Cat: 0x7fa450> (entity: Cat; id: 0x7b71f0 <x-coredata://7C904BD2-16AA-486D-8D1B-C2D0ABCCB6D4/Cat/p1> ; data: <fault>) cat id: (null) 

Since the cat id is null, the application crashes when I try to do something with it.

Therefore, the cat identifier is never null when I first retrieve the cat objects from CoreData and lay out the view. This is when I later touch one of the thumbnails that the data has become an error, so when I access the cat_id property, it is zero. CatThumbnailView saves the Cat object:

 @property (nonatomic, strong) Cat* cat; 

Why is this happening, and what should I do about it?

* note - this does not happen every time I click a thumbnail. I would say that this happens in 10-20% of cases. Other 80-90% of the time data is not an error. Why????

Answer 1

I know this is a little old, but the most likely explanation is that your implementation of your Cat class looked something like this:

 @implementation Cat @synthesise cat_id = _cat_id; ... @end 

When it should look like this:

 @implementation Cat @dynamic cat_id; ... @end 

I just made this mistake myself, so share it in the hope that it will save the next person a little time!

Answer 2

The master data does not return the complete object until it is necessary to access the actual value of this object. Each of your returned objects will be an “error” to this point.

You can use [request setReturnsObjectsAsFaults:NO] in a Fetch Request to force a complete object, but this is not necessary in most cases .

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In your case, I suspect that manageObjectContext is some way that was reached to an invalid state or inaccessible while fetching objects.

Instead of restricting the scope of manageObjectContext your manageObjectContext method, declare manageObjectContext at at the controller level so that it is available during the actual selection of the object.

PS Make sure that manageObjectContext is available during the actual selection of the object.

Hope this helps you.