NSSortDescriptor to-many relationships

问题

I use Core Data database. I have Entity Album, that has many entities SpecificImage (key to get set of these images is @"specificImages") I want to get Album with id = 1 and to sort attached SpecificImages by id. I tried

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

but I get the error

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here'

Here is my code:

    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];

it throws an exception, when I try to execute NSArray *objects = [context executeFetchRequest:request error:nil];

回答1:

I am assuming, if you have many SpecificImages in Album then you will also have an inverse relationship from SpecificImage to Album ( 1 Album object in SpecificImage ).

Here I am first finding all SpecificImages for the required album id rather than all Albums and in the end form one of those SpecificImage object I will get the album object back.

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];

The specificImagesArray is the array of specificImages you are looking for and album is your required album with id = CurrentCategoryItemID.

回答2:

I think, there is no any variant, how to do this. I had this problem. But I had to sord it by id after executing fetch request.

来源：https://stackoverflow.com/questions/10813826/nssortdescriptor-to-many-relationships

标签

iphone

objective-c

one-to-many

nssortdescriptor