Core Data fault after deleting managed object

Question

Basically, I'm trying to access some properties of a after deleting it from the NSManagedObjectContext and saving the context. The problem is that, after saving the context, Core Data marks the object data as fault and, apparently, is unable to recover it.

I've created a sample project in order to replicate the issue, you can download it here. To illustrate, the following snippet:

City *city = [self.cities objectAtIndex:indexPath.row];
[self.managedObjectContext deleteObject:city];
if (![self.managedObjectContext save:&error]) {
    [self.managedObjectContext rollback];
    NSLog(@"Error: %@", error);
}else{
    NSLog(@"%@", city);
    // All properties of "city" are zeroed.
    // Saved. Update data sources and animate changes...
}

produces:

<City: 0x7fe1cbd3cba0> (entity: City; id: 0xd000000000040004 <x-coredata://C1E3D3D8-188D-41DE-B701-08AF6D3E8860/City/p1> ; data: {
    country = "0xd000000000080002 <x-coredata://C1E3D3D8-188D-41DE-B701-08AF6D3E8860/Country/p2>";
    name = Rosario;
})

<City: 0x7fe1cbd3cba0> (entity: City; id: 0xd000000000040004 <x-coredata://C1E3D3D8-188D-41DE-B701-08AF6D3E8860/City/p1> ; data: <fault>)

The reason I'd like to access the managed object, after deleting it, is to update a NSMutableArray which acts as data source for a table view and update another data source in a previous controller in the navigation controller stack (This is not implemented in the sample project).

Wrapping up, my questions are:

After deleting a NSManagedObject from its NSManagedObjectContext and saving the context, it is no longer guaranteed that the data in the managed object will be accessible? Even if a reference to that managed object is kept?

Based on what i've researched, Core Data is getting rid of the entity data to save memory once the context is saved. Is this assumption correct? Are there other factors that might be causing this data faulting?

Thanks.

Why would you expect to be able to access a deleted object after saving the context? Use `NSFetchedResultsController` for data sources. — Avi, Jul 10 '16 at 09:59
I got confused because in a similar scenario I was able to access the data of a managed object after deleting it and saving the context. — Mateus, Jul 10 '16 at 18:35

adonoho · Accepted Answer · 2016-07-10T22:53:43.437

An NSManagedObject is always dynamically rendered. Hence, if it is deleted, Core Data faults out the data. It doesn't exist anymore. Your real question is how to remove an object from your various arrays? First, you should remove it before you delete the object using whichever search techniques you wish. This is the easiest and most robust path. Second, the object pointer itself is still valid and can be used with a -removeObject: call. Allow me to emphasize though, this is a fragile solution. I'm strongly encouraging you to remove the object before deleting it.

In answer to your second question,

Are there other factors that might be causing this data faulting?

No. Deleting the object is causing the faulting. If the data has heretofore been available, that is due to it being an implementation characteristic. Writing to the implementation instead of the spec, especially with database technologies, is fraught with all sorts of life cycle problems. Quoting the wise Doctor, "Don't do that."

Thanks! Could you elaborate why it is discouraged to remove the object after deleting it and making sure the context was saved? — Mateus, Jul 10 '16 at 18:42
Yes, the context save almost never fails. Basically, it fails with a very rare disk outage. iOS is well and truly screwed if you run out of space. It warns the user early. If they ignore the problem, they probably could not have launched your app. Second, you should regard a deleted object as a zombie. Only you know you sent a zombie to another method. It is expecting a live object. Hence, you are breaking your contract with the callee. You should expect the callee to break. — adonoho, Jul 10 '16 at 22:52

Core Data fault after deleting managed object

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