核心数据对多关系获取

Question

我在核心数据中设置了以下内容：包含许多成分的 Dish 实体和包含一个菜的成分实体。

我无法获取我的一对多关系数据，即某道菜的配料。我正在尝试使用与每道菜相关的成分填充我的收藏视图单元格。

我的核心数据模型设置如下：

我尝试了各种方法来获取数据。

我尝试过的事情之一是执行以下操作：Dish *dish = [self.fetchedResultsController objectAtIndexPath:indexPath];，然后是 NSLog disc.ingredients（它返回一个 NSSet），但返回整个核心数据项，即 createdAt 日期、名称以及所有内容，以及该菜的未格式化版本的配料。

我尝试的另一件事是使用Ingredient *ingredient = [dish.ingredients valueForKeyPath:@"ingredientName"]; 创建成分实体的新实例，但这会返回一个未格式化的字符串（我希望它在显示之前对其进行 UTF8 编码，因为它包含特殊字符）。

谷歌搜索时有很多解决方案建议使用 NSPredicate，但我就是不知道如何使用它。我对核心数据和 XCODE 还是很陌生，所以这里的任何帮助都将不胜感激，即你在哪里实现 NSPredicate，在我的情况下正确的谓词是什么？

一点支持代码：

我的 collectionViewController：

- (NSFetchedResultsController *)fetchedResultsController
{
    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    // Edit the entity name as appropriate.
    NSManagedObjectContext* context = [RKObjectManager sharedManager].managedObjectStore.mainQueueManagedObjectContext;
    self.managedObjectContext = context;
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Dish" inManagedObjectContext:self.managedObjectContext];
    [fetchRequest setEntity:entity];

    // Set the batch size to a suitable number.
    [fetchRequest setFetchBatchSize:20];

    // Edit the sort key as appropriate.

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"createdAt" ascending:NO];
    NSArray *sortDescriptors = @[sortDescriptor];

    [fetchRequest setSortDescriptors:sortDescriptors];

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Master"];
    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error]) {
        // Replace this implementation with code to handle the error appropriately.
        // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

    return _fetchedResultsController;
}

cellForItemAtIndexPath:

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    FOFPhotoCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"photo" forIndexPath:indexPath];

    self.object = [self.fetchedResultsController objectAtIndexPath:indexPath];
    Dish *dish = [self.fetchedResultsController objectAtIndexPath:indexPath];

    Ingredient *ingredient = ???;
    cell.backgroundColor = [UIColor lightGrayColor];





    [cell.imageView setImageWithURL:[NSURL URLWithString:[[NSString stringWithFormat:@"http://192.168.0.3:4000%@", dish.dishImage] description]] completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType) {
    }];

    cell.titleLabel.text = dish.dishName;
    cell.headerLabel.text = dish.dishHeader;


    cell.ingredientsLabel.text = [NSString stringWithFormat:@"%@", nil];

    NSLog(@"%@", ingredient);


    [self.collectionView reloadItemsAtIndexPaths:[NSArray arrayWithObject:indexPath]];

    return cell;
}

Answer 1

简单的方法是你的第一种方法，然后只使用你关心的成分的属性。 Core Data 更多的是关于对象编程而不是底层数据库，因此尝试模拟 DB 查询通常不是正确的方法。

我还建议去掉所有 valueForKey 的东西并使用实际的对象属性，这样代码会更清晰，编译器可以在数据类型方面为您提供更多帮助。

Answer 2

如果您只为一种特定的菜肴寻找成分，您必须为成分（而不是菜肴）设置您的 fetchRequest，并在菜肴上设置谓词。要使以下代码正常工作，您可能需要首先创建 NSManagedObject 子类（借助 Xcode 编辑器按钮）。

#import "Dish.h"
#import "Ingredient.h"

- (NSFetchedResultsController *)fetchedResultsController
{
    if (_fetchedResultsController != nil) {
        return _fetchedResultsController;
    }

    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Ingredient"];
    [fetchRequest setFetchBatchSize:20];

    //if dishname are unique values, set your predicate on it.
    //You can also set your predicate on the dishID if dishName are not unique values (several dishes with a same name)
    NSString *theDishIWant = @"Ratatouille"; //whatever Dish name you want
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"dish.dishName == %@", theDishIWant];
    [fetchRequest setPredicate:predicate];

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

    NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:[RKObjectManager sharedManager].managedObjectStore.mainQueueManagedObjectContext sectionNameKeyPath:nil cacheName:nil];

    aFetchedResultsController.delegate = self;
    self.fetchedResultsController = aFetchedResultsController;

    NSError *error = nil;
    if (![self.fetchedResultsController performFetch:&error]) {
        NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
        abort();
    }

NSLog (@"%@", _fetchedResultsController);
return _fetchedResultsController;
}

您的configureCell: atIndexPath: 方法可能如下所示：

- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
    Ingredient *ingredient = [self.fetchedResultsController objectAtIndexPath:indexPath];

    cell.textLabel.text = ingredient.ingredientName;
    cell.detailTextLabel.text = ingredient.ingredientValue;
}