FRC for Dependent entities.m

extern NSManagedObjectContext *GetContext(void);

@interface TableViewController () <NSFetchedResultsControllerDelegate>

@property (nonatomic) NSFetchedResultsController *controller;

@end

@implementation TableViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Organization"];
    request.sortDescriptors = @[ [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES] ];
    request.fetchBatchSize = 10;
    
    self.controller = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:GetContext() sectionNameKeyPath:nil cacheName:nil];
    self.controller.delegate = self;

    [[NSNotificationCenter defaultCenter] addObserverForName:NSManagedObjectContextDidSaveNotification object:self.controller.managedObjectContext.parentContext queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *note) {
        [self.controller.managedObjectContext mergeChangesFromContextDidSaveNotification:note];

        NSDictionary *userInfo = note.userInfo;
        NSArray *items = [@[ userInfo[NSUpdatedObjectsKey] ?: [NSSet set], userInfo[NSInsertedObjectsKey] ?: [NSSet set], userInfo[NSDeletedObjectsKey] ?: [NSSet set] ] valueForKeyPath:@"@unionOfSets.self"];

        [(NSManagedObjectContext *)note.object performBlock:^{
            NSMutableSet *objectIDs = [NSMutableSet set];
            for (NSManagedObject *object in items) {
                if ([object isKindOfClass:[Employee class]]) {
                    [objectIDs addObject:((Employee *)object).organization.objectID];
                }
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                for (NSManagedObjectID *objectID in objectIDs) {
                    Organization *organization = (Organization *)[self.controller.managedObjectContext objectWithID:objectID];
                    organization.name = organization.name; // <--- Magic is here!
                }
            });
        }];
    }];
    
    __autoreleasing NSError *error;
    [self.controller performFetch:&error];
    if (error) {
        NSLog(@"Fetch failed");
    }
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"TheCell" forIndexPath:indexPath];
    if (cell) {
        Organization *organization = [self.controller objectAtIndexPath:indexPath];
        cell.textLabel.text = organization.name;
        cell.detailTextLabel.text = [[organization valueForKeyPath:@"employees.@max.salary"] stringValue];
    }
    
    return cell;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return [self.controller.sections[section] numberOfObjects];
}

- (IBAction)addOrganizationTapped:(UIBarButtonItem *)sender {
    Organization *organization = [NSEntityDescription insertNewObjectForEntityForName:@"Organization" inManagedObjectContext:self.controller.managedObjectContext];
    organization.name = [NSString stringWithFormat:@"Organization %u", arc4random()];
    [organization addEmployees:[NSSet setWithObjects:[self employeeWithID:1], [self employeeWithID:2], nil]];

    __autoreleasing NSError *saveError;
    if (![self.controller.managedObjectContext save:&saveError]) {
        NSLog(@"Save error: %@", saveError);
    }
}

- (Employee *)employeeWithID:(NSUInteger)ID {
    Employee *result = [NSEntityDescription insertNewObjectForEntityForName:@"Employee" inManagedObjectContext:self.controller.managedObjectContext];
    result.name = [NSString stringWithFormat:@"Employee %tu", ID];
    result.salary = arc4random();
    return result;
}

- (IBAction)increaseSalaryTapped:(UIBarButtonItem *)sender {
    NSUInteger numberOfOrganizations = [self.controller.sections.firstObject numberOfObjects];
    Organization *organization = [self.controller objectAtIndexPath:[NSIndexPath indexPathForRow:numberOfOrganizations - 1 inSection:0]];
    NSManagedObjectID *maxSalaryEmployeeID = [[organization.employees sortedArrayUsingDescriptors:@[ [NSSortDescriptor sortDescriptorWithKey:@"salary" ascending:NO] ]].firstObject objectID];
    
    NSManagedObjectContext *privateContext = self.controller.managedObjectContext.parentContext;
    [privateContext performBlock:^{
        Employee *maxSalaryEmployee = (Employee *)[privateContext objectWithID:maxSalaryEmployeeID];
        int32_t delta = arc4random() % 1000;
        maxSalaryEmployee.salary += delta;
        
        __autoreleasing NSError *saveError;
        if (![privateContext save:&saveError]) {
            NSLog(@"Save error: %@", saveError);
        }
    }];
}

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView beginUpdates];
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
    [self.tableView endUpdates];
}

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
    UITableViewRowAnimation animation = UITableViewRowAnimationAutomatic;
    switch (type) {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertRowsAtIndexPaths:@[ newIndexPath ] withRowAnimation:animation];
            break;
        case NSFetchedResultsChangeUpdate:
            [self.tableView reloadRowsAtIndexPaths:@[ indexPath ] withRowAnimation:animation];
            break;
        case NSFetchedResultsChangeDelete:
            [self.tableView deleteRowsAtIndexPaths:@[ indexPath ] withRowAnimation:animation];
            break;
        case NSFetchedResultsChangeMove:
            [self.tableView moveRowAtIndexPath:indexPath toIndexPath:newIndexPath];
            break;
    }
}