Question

我使用Core Data数据库。我有Entity Album，它有许多实体SpecificImage（获取这些图像集的关键是@“specificImages”）我希望获得id = 1的Album并按id排序附加的SpecificImages。我试过了

NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"specificImages.id" ascending:YES];

但我收到了错误

由于未捕获的异常而终止应用 'NSInvalidArgumentException'，原因：'这里不允许使用多对键'

这是我的代码：

    NSString *entityName = kEntityName;
    AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];;
    NSManagedObjectContext *context = appDelegate.managedObjectContext;
    NSEntityDescription *entityDesctiption = [NSEntityDescription 
                                              entityForName: entityName
                                              inManagedObjectContext:context];

    // find object
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"id == %d", CurrentCategoryItemID];
    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"specificImages.id" ascending:YES];
    [request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
    [request setEntity:entityDesctiption];
    [request setPredicate:predicate];
    NSArray *objects = [context executeFetchRequest:request error:nil];
    [request release];
    if (objects == nil)
    {
        NSLog(@"there was an error");
        return;
    }
    else if ([objects count] == 0)
    {
        return;
    }
    NSManagedObject *object = [objects objectAtIndex:0];

当我尝试执行NSArray * objects = [context executeFetchRequest：request error：nil];

时抛出异常

Answer 1

我假设，如果您在相册中有许多特定图像，那么您还将具有从特定图像到相册的反向关系（特定图像中的1个相册对象）。

这里我首先找到所有专辑ID的所有ID，而不是所有专辑，最后形成其中一个SpecificImage对象，我将获得专辑对象。

NSString *entityName = @"SpecificImage";
AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];;
NSManagedObjectContext *context = appDelegate.managedObjectContext;
NSEntityDescription *entityDesctiption = [NSEntityDescription 
                                          entityForName: entityName
                                          inManagedObjectContext:context];

// find object
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"album.id == %d", CurrentCategoryItemID];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"id" ascending:YES];
[request setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];
[request setEntity:entityDesctiption];
[request setPredicate:predicate];
NSArray *specificImagesArray = [context executeFetchRequest:request error:nil];
[request release];
if (objects == nil)
{
    NSLog(@"there was an error");
    return;
}
else if ([objects count] == 0)
{
    return;
}
Album *album = [(SpecificImage *)[specificImagesArray objectAtIndex:0] album];

specificImagesArray是您要查找的特定图像的数组，相册是您所需的专辑，其ID = CurrentCategoryItemID。

Answer 2

我认为，没有任何变体，如何做到这一点。我有这个问题。但是我必须在执行获取请求后通过id对其进行处理。

NSSortDescriptor to-many关系

2 个答案: