ios - 从核心数据中的一对多关系中删除对象

Question

我目前在删除一对多关系中的对象时遇到问题。

我的应用有以下关系:

产品<<- 购物车

当用户在我的 View Controller 中按下“添加到购物车”按钮时，以下代码设置产品对象和购物车之间的关系

    + (Cart *)addProductToCartWithProduct:(Product *)product inManagedObjectContext:(NSManagedObjectContext *)context {

    Cart *cart = nil;

    NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:@"Cart"];

    NSError *error = nil;
    NSArray *carts = [context executeFetchRequest:request error:&error];

    if (!carts || ([carts count] > 1)) {
        // handle error
    } else if (![carts count]) {
        cart = [NSEntityDescription insertNewObjectForEntityForName:@"Cart" inManagedObjectContext:context];


    } else { // they already have a cart started
        cart = [carts lastObject];
    }
    /*Get Object ID to safely pass NSMangedObject between threads (A background worker thread and the main thread). */
      NSManagedObjectID *retID = [product objectID];
    [cart addProductsObject:(Product *)[context objectWithID:retID]];

//Inverse relationship
    [(Product *) [context objectWithID:retID] setInCart:cart];


     return cart;
}

这然后返回一个购物车对象，我将其传递给我的购物车 View Controller ，并像这样获取该关系中的产品:

             // Fetch request for "Product":
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Product"];

// Fetch only products for the cart:
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"inCart = %@", self.cart];
[fetchRequest setPredicate:predicate]; 


NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"navn" ascending:YES];
[fetchRequest setSortDescriptors:@[sortDescriptor]];

self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_theManagedObjectContext sectionNameKeyPath:nil cacheName:nil];
_fetchedResultsController.delegate = self;

然后当我尝试从这样的关系中删除一个对象时:

-(void)RemoveFromCart:(UIButton *)sender {

     NSIndexPath *ip = [NSIndexPath indexPathForRow:sender.tag inSection:0];

    Product *prod = (Product *)[self.fetchedResultsController objectAtIndexPath:ip];



       prod.inCart = nil;
       [_cart removeProductsObject:prod];


    NSLog(@"Cart %@ %@", _cart.products, prod);


    [self saveCurrentContext:_theManagedObjectContext];




    [self loadCart]; 
    [_orderTable reloadData];


}

产品在视觉上被移除(从 tableview/screen 中消失)因为 inCart 设置为 nil，但技术上不是......当我记录购物车对象时，产品对象仍在关系中，所以看起来像[_cart removeProductsObject:prod]; 不工作。

反之亦然，当我尝试将相同的产品添加到购物车时，我只是(从购物车中)删除了，出于某种原因，未设置反向关系“inCart”，在我将其设置为“nil”之后，当产品对象被删除时。

为什么会这样？我该如何解决？ :).

编辑:

在核心数据模型编辑器中展示反向关系的图片:

将购物车传递给其他 View Controller :

[[[DataManager sharedInstance] backgroundManagedObjectContext] performBlock:^{


    UITabBarController *tabBarController = self.tabBarController; 

    for (UINavigationController *navController in  tabBarController.viewControllers) {


        for (UIViewController *vc in navController.viewControllers) {
            if ([vc isMemberOfClass:NSClassFromString(@"CartViewController")]){ 
               CartViewController *cartVC = (CartViewController *) vc;
                cartVC.cart = [Cart addProductToCartWithProduct:prod inManagedObjectContext:         [[DataManager sharedInstance] backgroundManagedObjectContext]];

                [[DataManager sharedInstance] saveBackgroundContext];
                [[DataManager sharedInstance] saveMasterContext];

                NSLog(@" %@", cartVC.cart); 
            }
        }

    }



}];

保存上下文

-(void)saveCurrentContext:(NSManagedObjectContext *)context  {

    NSError *error = nil;
    if (![context save:&error]) {
        NSLog(@"NOT SAVED");
    }
    [[DataManager sharedInstance] saveBackgroundContext];
    [[DataManager sharedInstance] saveMasterContext];



}

Answer 1

好的，所以我找到了解决这个非常奇怪的问题的方法。由于自动生成的访问器方法由于某种原因不起作用，我不得不想出另一种方法来从关系中删除对象。

          for (Product *prod in _cart.products) {

//To Very reliable to check for item by name attribute, but It works :) 
              if ([prod.name isEqualToString:product.name]) {

                        product = prod; 

       NSMutableSet *set = [NSMutableSet setWithSet:_cart.products];
       [set removeObject:prod];
       _cart.products = set;


              }


            }