I have an app based on the CoreDataBooks example that uses an addingManagedObjectContext to add an Ingredient to a Cocktail in order to undo the entire add. The CocktailsDetailViewController in turn calls a BrandPickerViewController to (optionally) set a brand name for a given ingredient. Cocktail, Ingredient and Brand are all NSManagedObjects. Cocktail requires at least one Ingredient (baseLiquor) to be set, so I create it when the Cocktail is created.

If I add the Cocktail in CocktailsAddViewController : CocktailsDetailViewController (merging into the Cocktail managed object context on save) without setting baseLiquor.brand, then it works to set the Brand from a picker (also stored in the Cocktails managed context) later from the CocktailsDetailViewController.

However, if I try to set baseLiquor.brand in CocktailsAddViewController, I get:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Illegal attempt to establish a relationship 'brand' between objects in different contexts'

From this question I understand that the issue is that Brand is stored in the app's managedObjectContext and the newly added Ingredient and Cocktail are stored in addingManagedObjectContext, and that passing the ObjectID instead would avoid the crash.

What I don't get is how to implement the picker generically so that all of the Ingredients (baseLiquor, mixer, garnish, etc.) can be set during the add, as well as one-by-one from the CocktailsDetailViewController after the Cocktail has been created. In other words, following the CoreDataBooks example, where and when would the ObjectID be turned into the NSManagedObject from the parent MOC in both add and edit cases? -IPD

UPDATE - Here's the code:

- (IBAction)addCocktail:(id)sender {

    CocktailsAddViewController *addViewController = [[CocktailsAddViewController alloc] init];
    addViewController.title = @"Add Cocktail";
    addViewController.delegate = self;

    // Create a new managed object context for the new book -- set its persistent store coordinator to the same as that from the fetched results controller's context.
    NSManagedObjectContext *addingContext = [[NSManagedObjectContext alloc] init];
    self.addingManagedObjectContext = addingContext;
    [addingContext release];

    [addingManagedObjectContext setPersistentStoreCoordinator:[[fetchedResultsController managedObjectContext] persistentStoreCoordinator]];

    Cocktail *newCocktail = (Cocktail *)[NSEntityDescription insertNewObjectForEntityForName:@"Cocktail" inManagedObjectContext:self.addingManagedObjectContext];
    newCocktail.baseLiquor = (Ingredient *)[NSEntityDescription insertNewObjectForEntityForName:@"Ingredient" inManagedObjectContext:self.addingManagedObjectContext];
    newCocktail.mixer = (Ingredient *)[NSEntityDescription insertNewObjectForEntityForName:@"Ingredient" inManagedObjectContext:self.addingManagedObjectContext];
    newCocktail.volume = [NSNumber numberWithInt:0];
    addViewController.cocktail = newCocktail;

    UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:addViewController];

    [self.navigationController presentModalViewController:navController animated:YES];

    [addViewController release];
    [navController release];

}