iphone - 核心数据一对多关系

我的核心数据模型中存在一对多关系。一顿饭可以有多种食物。我想在一张桌子上放满一顿饭的食物。因此，如果第一餐中有苹果，橙子和柠檬，则餐桌上将包含这三种食物。

非常感谢您的帮助，因为我完全被卡住了。

这是我的模型:

和NSManagedObject类:

餐时

@class Food;

@interface Meal : NSManagedObject

@property (nonatomic, retain) NSString *name;
@property (nonatomic, retain) NSSet *foods;

@end

@interface Meal (CoreDataGeneratedAccessors)

- (void)addFoodsObject:(Food *)value;
- (void)removeFoodsObject:(Food *)value;
- (void)addFoods:(NSSet *)values;
- (void)removeFoods:(NSSet *)values;

@end

食物

@class Meal;

@interface Food : NSManagedObject

@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) Meal *meals;

@end

这是我尝试尝试的代码，但是我遇到了一些问题。

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

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Food" inManagedObjectContext:_context];
    [fetchRequest setEntity:entity];

    [fetchRequest setPredicate:[NSPredicate predicateWithFormat:@"Meal.foods == %@", entity]];

    NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"name" ascending:NO];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
    [fetchRequest setFetchBatchSize:20];

    NSFetchedResultsController *theFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:_context sectionNameKeyPath:nil cacheName:@"Root"];

    self.fetchedResultsController = theFetchedResultsController;
    _fetchedResultsController.delegate = self;

    [fetchRequest release];
    [theFetchedResultsController release];

    return _fetchedResultsController;
}

最佳答案

您提到自己遇到了问题，但不是问题所在。但是您的NSFetchRequest谓词对我而言并不正确。同样，在meals上定义的Food是复数形式，但关系是一对一-有点令人困惑。

您的UITableView应该使用单个Food中的Meal填充，对吗？让我们将其称为Meal _thisMeal，然后获取谓词应为:

NSPredicate *foodPredicate = [NSPredicate predicateWithFormat:@"meals == %@",_thisMeal];
[fetchRequest setPredicate:foodPredicate];

因此，您要获取的实体是Food；并且此谓词可确保其meals属性是您的UITableViewController要查找的Meal。

更新以显示如何获取命名的Meal
要获取名为“自定义餐”的Meal，请执行以下操作:

Meal *_thisMeal = nil;
// this assumes that there should be only one Meal named "Custom Meal"
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"Meal"];
NSPredicate *fetchPredicate = [NSPredicate predicateWithFormat:@"name = %@",@"Custom Meal"];

NSError *fetchError = nil;
NSArray *meals = [_context executeFetchRequest:request error:&fetchError];
if( fetchError )
    NSLog(@"%s - ERROR while fetching Meal: %@,%@",__FUNCTION__,fetchError,[fetchError userInfo]);
else if( [meals count] != 0 )
    _thisMeal = [meals objectAtIndex:0];