Question

I have two entities: products and bundles. Each one has its class. A product can be in multiple bundles.

Entities are defined like this:

PRODUCTS
name, string
number, integer 16
fromBundle = to-many relationship to product

BUNDLE
name, string
number, integer 16
product = to-many relationship to fromBundle

Products were assigned to bundle like this:

// suppose bundle 1 is composed of products 1, 2, 3 and 4.
NSArray *myProd = [NSArray arrayWithObjects:
    [NSNumber numberWithInt:1],
    [NSNumber numberWithInt:2],
    [NSNumber numberWithInt:3],
    [NSNumber numberWithInt:4],
    nil];

int bundleNumber = 1;
NSString *bundleName = @"My Bundle";
Bundle *aBundle = nil;

NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];

request.entity = [NSEntityDescription entityForName:@"Bundle" inManagedObjectContext:context];
request.predicate = [NSPredicate predicateWithFormat: @"(number == %d)", bundleNumber];
NSError *error = nil;
aBundle = [[context executeFetchRequest:request error:&error] lastObject];

// as the bundle does not exist, this will run
if (!error && !aBundle) {
    aBundle = [NSEntityDescription insertNewObjectForEntityForName:@"Bundle" inManagedObjectContext:context];
    aBundle.string = bundleName;
    aBundle.Number = [NSNumber numberWithInt:bundleNumber];

    for (NSNumber *umNum in myProd) {

            // the product with number = aNum is retrieved... yes it is valid at this point 
            Product *oneProduct = [ProductWithNumber:umNum inManagedObjectContext:context];
            NSMutableSet *mutableSet = [oneProduct mutableSetValueForKey:@"fromBundle"];
            [mutableSet addObject:aBundle];
    }
// Save the context.
NSError *error = nil;
if (![context save:&error]) {
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
}

// everything is fine at this point.

Now I wish to retrieve a list of all products that belong to a specific bundle…

To do that, I am using this method on Bundle class

+ (NSArray *)ProductsInBundle:(Bundle*)aBundle inManagedObjectContext:(NSManagedObjectContext *)context
{
    NSArray *all = nil;


    NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
    request.entity = [NSEntityDescription entityForName:@"Products" inManagedObjectContext:context];
    request.predicate = [NSPredicate predicateWithFormat:@"(fromBundle == %@)", aBundle];

    NSError *error = nil;
    all = [context executeFetchRequest:request error:&error]; // crashes here

    return all;
}

it crashes on the assigned line on the last method with the message “to-many key not allowed here” when I try to do this

NSArray *allProductsInBundle = [Bundle ProductsInBundle:aBundle inManagedObjectContext:self.managedObjectContext];

aBundle is valid at this point.

Answer 1

I think your predicate is wrong. You don’t have a bundle property, but a fromBundle property.

If it is really fromBundle, then your predicate should be:

equest.predicate = [NSPredicate predicateWithFormat:@"(fromBundle == %@)", aBundle];

EDIT:

If you are trying to do operations on to-many relationships then you’ll need to use the aggregate functions for the predicate. I think for your case you’ll want the IN operation.

http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/Predicates/Articles/pSyntax.html#//apple_ref/doc/uid/TP40001795-215891