Core Data: Keypath "objectID" not found in entity

Question

I'm using NSFetchedResultsController with a predicate to load a list of Documents in my application. I want to load all the Documents except the currently active one.

I am using Rentzsch's MOGenerator to create a _Document class and then I put all my custom code in the Document subclass. _Document generates an objectID property with type DocumentID.

In the class that creates the controller, I set the controller's currentDocID property:

controller.currentDocID = self.document.objectID;

In the controller itself, I lazy load the fetchedResultsController like this:

- (NSFetchedResultsController *)fetchedResultsController {
    if (fetchedResultsController != nil) {
        return fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Document" inManagedObjectContext:managedObjectContext];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(objectID != %@)", self.currentDocID];
    [fetchRequest setPredicate:predicate];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"dateModified" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];

    [fetchRequest setSortDescriptors:sortDescriptors];

    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:@"Root"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    [aFetchedResultsController release];
    [sortDescriptor release];
    [sortDescriptors release];

    return fetchedResultsController;
}

When the fetchedResultsController loads, my app crashes with an unhandled exception:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'keypath objectID not found in entity <NSSQLEntity Document id=1>'

It's my understanding that all NSManagedObjects have an objectID, whether temporary or permanent. Is this not the case? Any thoughts?

Solution

Change your predicate to read

[NSPredicate predicateWithFormat:@"self != %@", [self currentDoc]]

Where currentDoc is a reference to the instance of the NSManagedObject that represents the current document.

Core Data will do the equality check internally.

Comments

[Lindemann]

Do you know why ...@"self != %@", [self currentDoc]] is not the same like ...@"(objectID != %@)", self.currentDocID]. According to this post stackoverflow.com/a/2306550/647644 it should be equivalent.