Core Data relationships issue

Question

I've got a simple app with three views HomeViewController , AddViewController , and AddCategoryViewController .

My data model has two Entities, with relationships like this:

My goal is to be able to display WMDGActivity objects in the tableview in HomeViewController , grouped under sections defined by WMDGCategory objects.

New WMDGCategory objects are added via a textfield in AddCategoryViewController and new WMDGActivity objects are added via a textfield in AddViewController .

My problem is that the app crashes upon canceling or saving in either of the auxiliary views. The error code is invariably:

reason: '[<WMDGActivity 0x8a54830> valueForUndefinedKey:]: the entity WMDGActivity is not key value coding-compliant for the key "WMDGCategory".'

I also see this every time I roll the picker wheel containing the names of WMDGCategory objects:

data: {
    activities = "<relationship fault: 0x8db1600 'activities'>";
    name = Pastimes;

Indeed, another mystery is that the app is apparently saving these objects before the crash. I've tried stepping through the code, but haven't come up with a solution yet. I suspect there is a problem with the way my entities are set up, or with my code that interacts with them.

Here is what I believe to be the relevant code:

From HomeViewController (delegate for both AddViewController , and AddCategoryViewController :

#pragma mark AddViewControllerDelegate stuff

-(void) addViewControllerDidSave
{
    NSManagedObjectContext *localContext = [NSManagedObjectContext MR_contextForCurrentThread];
    [localContext MR_saveToPersistentStoreAndWait];
    [self.navigationController dismissViewControllerAnimated:YES completion:nil];
    [self refreshData];

}

-(void) addViewControllerDidCancel:(WMDGActivity *) activityToDelete
{
    [activityToDelete MR_deleteEntity];
    [self.navigationController dismissViewControllerAnimated:YES completion:nil];
    [self refreshData];

}

#pragma mark AddCatControllerDelegate stuff

-(void) addCatControllerDidSave
{
    NSManagedObjectContext *localContext = [NSManagedObjectContext MR_contextForCurrentThread];
    [localContext MR_saveToPersistentStoreAndWait];
    [self.navigationController dismissViewControllerAnimated:YES completion:nil];
    [self refreshData];

}

-(void) addCatControllerDidCancel:(WMDGCategory *) categoryToDelete
{
    [categoryToDelete MR_deleteEntity];
    [self.navigationController dismissViewControllerAnimated:YES completion:nil];
    [self refreshData];

}

From AddCategoryViewController:

- (IBAction)saveButton:(UIBarButtonItem *)sender
{
    if (self.catTextField.text.length > 0)
    {
        self.thisCategory.name = self.catTextField.text;
    }
    [self.delegate addCatControllerDidSave];
}

- (IBAction)cancelButton:(UIBarButtonItem *)sender
{
    [self.delegate addCatControllerDidCancel:self.thisCategory];
}

And from AddViewController:

- (IBAction)saveButton:(UIBarButtonItem *)sender
{
    if (self.activityField.text.length > 0)
    {

        if (self.categoryLabel.text.length < 1)
        {
            self.thisCategory.name = @"Uncategorized";
            //            self.thisActivity.activityName = self.activityField.text;
            //            [self.delegate addActivityViewControllerDidSave];
        }

        else
        {
        self.thisCategory.name = self.categoryLabel.text;
        self.thisActivity.name = self.activityField.text;
        NSLog(@"Category name is %@", self.thisCategory.name);
        NSLog(@"Activity name is %@", self.thisActivity.name);
        }


        [self.delegate addViewControllerDidSave];

    }

    else
    {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"No activity entered"
                                                        message:@"Please enter a new activity or Cancel"
                                                       delegate:nil
                                              cancelButtonTitle:@"OK"
                                              otherButtonTitles:nil];
        [alert show];
    }



}

- (IBAction)cancelButton:(UIBarButtonItem *)sender
{
    [self.delegate addViewControllerDidCancel:self.thisActivity];
}

Can anyone please show me what I'm doing wrong?

Answer 1

My problem is that the app crashes upon canceling or saving in either of the auxiliary views. The error code is invariably:

It would help to see the error call stack and the code where the error occurred. But I'll go ahead and throw out a guess, which is that you're configuring an NSFetchedResultsController and specifying WMDGCategory as the sectionNameKeyPath . There are two things wrong with this. First, the key path is toCategory . WMDGCategory is the data type. Second, to organize your table view by category name, you would need to use the key path toCategory.name . Also, you're going to need to pre-sort your activities by toCategory.name (a requirement of NSFetchedResultsController ).

I also see this every time I roll the picker wheel containing the names of WMDGCategory objects:

What you're seeing is the description of the WMDCategory object. What you need is the value of the name property. You need to use [category valueForKey:@"name"] , or if you're using convenience classes, category.name .