Question

I am creating an iOS 5 app using Core Data.

I have two entities Item and Category there is a many-to-many relationship between these two entities, i.e. an Item belongs to many categories and a Category has many items.

What I am trying to do is, given a category, I want to know all the categories the items of that category is associated with. For that I am trying to execute the following request:

NSEntityDescription *catEntity = [NSEntityDescription entityForName:@"Category" inManagedObjectContext:context];
NSFetchRequest *request = [[NSFetchRequest alloc] init];
request.entity = catEntity;
request.predicate = [NSPredicate predicateWithFormat:@"items IN %@",[category.items allObjects]];
request.sortDescriptors = [NSArray arrayWithObject:[NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES]];    
NSError *error = nil;
NSArray *categories = [context executeFetchRequest:request error:&error];

But when executing this I get the following error:

'NSInvalidArgumentException', reason: 'unimplemented SQL generation for predicate : (items IN {<Item: 0x89b9c50> (entity: Item; id: 0x89be960 <x-coredata://A5DE0832-95F1-460C-9F40-202A10E16BDC/Item/p17> ; data: {
    attributes =     (
    );
    categories =     (
        "0x89be540 <x-coredata://A5DE0832-95F1-460C-9F40-202A10E16BDC/Category/p28>",
        "0x89be680 <x-coredata://A5DE0832-95F1-460C-9F40-202A10E16BDC/Category/p29>"
    );
    name = "Some Item";
})})'

What am I doing wrong here?

Answer 1

Try this, I’m not sure but it might work:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"ANY items IN %@", [category.items allObjects]];