核心数据：获取一对多关系中的所有孩子

Question

我有一对多的关系，并且我成功地将数据放入其中。问题是我无法通过关系属性获取它。

我的添加代码：

   NSManagedObjectContext *context = [[CBICoreDataController sharedInstance] masterManagedObjectContext];
NSManagedObject *orders = [NSEntityDescription
                           insertNewObjectForEntityForName:@"UnsyncedOrders"
                           inManagedObjectContext:context];
   [orders setValue:[dictionary valueForKey:@"specialInstructions"] forKey:@"specialInstructions"];
[orders setValue:[dictionary valueForKey:@"poNumber"] forKey:@"poNumber"];
[orders setValue:[dictionary valueForKey:@"specialInstructions"] forKey:@"specialInstructions"];
 [orders setValue:[dictionary valueForKey:@"deliveryDate"] forKey:@"deliveryDate"];
 [orders setValue:[dictionary valueForKey:@"account"] forKey:@"account"];

NSMutableSet *muteSet=[orders mutableSetValueForKey:@"items"];

for (NSDictionary *dict in [dictionary valueForKey:@"items"]) {
    NSManagedObject *items = [NSEntityDescription
                              insertNewObjectForEntityForName:@"UnsyncedOrderItems"
                              inManagedObjectContext:context];
    [items setValue:[dict valueForKey:@"arraykey"] forKey:@"arrayKey"];
     //Other Sets Here

    [muteSet addObject:items];
}

[orders setValue:muteSet forKey:@"items"];

    NSError *error;
    if (![context save:&error]) {
        NSLog(@"Error saving in writeToMenuCostingDetailsTable: %@", [error localizedDescription]);
    }

我现在可以获取但它不起作用：

NSFetchRequest * fetchRequest = [[NSFetchRequest alloc] init];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"arrayKey" ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];

NSEntityDescription *parent = [NSEntityDescription entityForName:@"UnsyncedOrders"
            inManagedObjectContext:context];

NSEntityDescription *child = [NSEntityDescription entityForName:@"UnsyncedOrderItems"
            inManagedObjectContext:context];


[fetchRequest setEntity:child];

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"orders == %@", parent];
[fetchRequest setPredicate:predicate];

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:context sectionNameKeyPath:nil cacheName:nil];

NSError *error;

[aFetchedResultsController performFetch:&error];
NSArray *fetchedObjects = [aFetchedResultsController fetchedObjects];

Answer 1

您的orders 关系与UnsyncedOrders 的特定实例 相关。但是，当您尝试执行 fetch 时，您会这样做：

NSEntityDescription *parent = [NSEntityDescription entityForName:@"UnsyncedOrders"
        inManagedObjectContext:context];
[...]
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"orders == %@", parent];
[fetchRequest setPredicate:predicate];

您正在尝试获取orders 关系指向UnsyncedOrders 实体本身的实例，而不是那些与UnsyncedOrders 的特定实例相关的实例。那总是会失败的。事实上，如果您检查 performFetch: 的结果，很有可能它会返回 NO，并为您提供描述问题的某种错误。

如果要获取与UnsyncedOrders 的特定实例相关的UnsyncedOrderItems，则需要使用指向特定实例的谓词。但是，如果您已经拥有该实例，则无需获取 - 只需向 UnsyncedOrders 实例询问其 items 关系的值即可。

Answer 2

这是我最终使用的解决方案：

NSManagedObjectContext *context = [[CBICoreDataController sharedInstance] masterManagedObjectContext];
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"UnsyncedOrders"
                                          inManagedObjectContext:context];
NSError *error = nil;
[fetchRequest setEntity:entity];
NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:&error];
for (UnsyncedOrders *info in fetchedObjects) {
    [mutableArray addObject:info.items];
}