Removing an Object with A Certain Title from an NSTreeController.

Question

I am wondering how I could delete an object depending on it's title for the CoreData 'name' property I have. To Add an Object I use this code:

NSManagedObjectContext *moc = [self managedObjectContext];
JGManagedObject *theParent = 
    [NSEntityDescription insertNewObjectForEntityForName:@"projects"
                                  inManagedObjectContext:moc];
[theParent setValue:nil forKey:@"parent"];
// This is where you add the title from the string array
[theParent setValue:@"myTitle" forKey:@"name"]; 
[theParent setValue:[NSNumber numberWithInt:0] forKey:@"position"];

But I can't seem to find an equivalent function to remove An object.

Answer 1

I don't know if you looked in the Core Data Programming guide in the section for adding and deleting objects.

Edit

I've modified this to delete from an array of names. Again; less then 5 minutes work with the Predicate Programming Guide.

- (void)removeObjectsWithNames:(NSArray *)nameArray {
    // Get the moc and prepare a fetch request for the required entity
    NSManagedObjectContext *moc = [self managedObjectContext];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"Project" inManagedObjectContext:moc];
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:entityDescription];

    // Create a predicate for an array of names.
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name IN %@", nameArray];
    [request setPredicate:predicate];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
    [request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];

    // Execute the fetch request put the results into array
    NSError *error = nil;
    NSArray *resultArray = [moc executeFetchRequest:request error:&error];
    if (resultArray == nil)
    {
        // Diagnostic error handling
        NSAlert *anAlert = [NSAlert alertWithError:error];
        [anAlert runModal];
    }

    // Enumerate through the array deleting each object.
    // WARNING, this will delete everything in the array, so you may want to put more checks in before doing this.
    For (JGManagedObject *objectToDelete in resultArray ) {
        // Delete the object.
        [moc deleteObject:objectToDelete];
    }
}

Edited 10/10/2009 - To add what Joshua has tried.

for(NSString *title in oldTasks) { // 1
    // Get the moc and prepare a fetch request for the required entity
    NSManagedObjectContext *moc = [self managedObjectContext];
    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:@"projects" inManagedObjectContext:moc];
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    [request setEntity:entityDescription];

    // Create a predicate for an array of names.
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"title IN %d", oldTasks]; // 2
    [request setPredicate:predicate];

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:YES];
    [request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];

    // Execute the fetch request put the results into array
    NSError *error = nil;
    NSArray *resultArray = [moc executeFetchRequest:request error:&error];
    if (resultArray == nil)
    {
        // Diagnostic error handling
        NSAlert *anAlert = [NSAlert alertWithError:error];
        [anAlert runModal];
    }

    JGManagedObject *objectToDelete = [resultArray objectAtIndex:0];
    // Delete the object.
    [moc deleteObject:objectToDelete];
}

Notes

I've highlighted two lines.

1. You've pasted my example as a for loop rather than a function call. This is just taking the strings off one at a time and passing them into the method. In my example, I'm passing in an array of strings that you want to match.

2. This is where you are having your problem. If you'd bothered reading the the Predicate Programming Guide, right at the top, in the Predicates Basics section it says it expects the class that it is being used with should be KVC compliant. This is why you are getting the error about KVC compliance. You are trying to search for title IN... but title isn't a property of your model.

I think you may be confused about what a predicate does. Look at the exaple code that I wrote.

Firstly, I create a Fetch request which will select objects from the 'Projects' entity.

Secondly, I create a predicate which says for each object returned by the fetch request, get the value of the 'name' property and compare it to the values of the objects in the 'namesArray'

Thirdly, I'm creating a sort descriptor that will sort the results in ascending order based on the 'name' property.

Then, once I've set this fetch request up, I run it against the moc and it returns an array of objects that match these criteria.