添加为关系核心数据设置 nil 的功能

Question

我有两个具有多对一关系的实体。例如Employee <<---> Shop。当我创建一个新员工时，我可以为它选择一个商店。我获取所有可用的商店，然后从表格视图中选择一个。
现在，我想在这个表中添加一个新行，以便能够将 nil 设置为关系，例如通过添加一个名为 "None" 的行，当它被选中时，关系将为 employee.shop = nil;
是否可以？我不知道如何配置表格视图来完成这项工作...
但是，这是用于获取商店的代码：

-(NSArray *)projectsList
{
    if (!projectsList) {
        NSFetchRequest *request = [[NSFetchRequest alloc] init];
        NSEntityDescription *entity = [NSEntityDescription entityForName:@"Project" inManagedObjectContext:taskObject.managedObjectContext];
        [request setEntity:entity];

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

        NSError *error = nil;
        NSArray *projects = [taskObject.managedObjectContext executeFetchRequest:request error:&error];
        if (!projects) {
            NSLog(@"Risultati della richiesta nulli!");
            abort();
        }

        projectsList = [projects mutableCopy];
    }

    return projectsList;
}

还有一些tableView方法：

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return [[self projectsList] count];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
    }

    Project *project = (Project *)[[self projectsList] objectAtIndex:indexPath.row];
    cell.textLabel.text = project.title;

    if (project == taskObject.project) {
        cell.accessoryType = UITableViewCellAccessoryCheckmark;
    }
    else {
        cell.accessoryType = UITableViewCellAccessoryNone;
    }

    return cell;
}

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];

    cell.accessoryType = UITableViewCellAccessoryCheckmark;

    Project *project = (Project *)[projectsList objectAtIndex:indexPath.row];
    taskObject.project = project;

    NSError *error = nil;
    if (![taskObject.managedObjectContext save:&error]) {
        NSLog(@"Errore nel salvare il progetto per il task! %@, %@", error, [error userInfo]);
        abort();
    }
    else {
        [self.navigationController popViewControllerAnimated:YES];
    }
}

非常感谢！

Answer 1

应该这样做。

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return [[self projectsList] count] + 1;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
    }

    if ( indexPath.row == [projectsList count] )
    {
        cell.textLabel.text = @"None";
        cell.accessoryType = ( taskObject.project ) ? UITableViewCellAccessoryNone : UITableViewCellAccessoryCheckmark;
    }
    else
    {
        Project *project = (Project *)[[self projectsList] objectAtIndex:indexPath.row];
        cell.textLabel.text = project.title;

        if (project == taskObject.project) {
            cell.accessoryType = UITableViewCellAccessoryCheckmark;
        }
        else {
            cell.accessoryType = UITableViewCellAccessoryNone;
        }
    }

    return cell;
}

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];

    cell.accessoryType = UITableViewCellAccessoryCheckmark;

    if ( indexPath.row == [projectsList count] )
    {
        taskObject.project = nil;
    }
    else
    {
        Project *project = (Project *)[projectsList objectAtIndex:indexPath.row];
        taskObject.project = project;
    }

    NSError *error = nil;
    if (![taskObject.managedObjectContext save:&error]) {
        NSLog(@"Errore nel salvare il progetto per il task! %@, %@", error, [error userInfo]);
        abort();
    }
    else {
        [self.navigationController popViewControllerAnimated:YES];
    }
}