Grouping three entities with many-to-many relationships - iphone

Question

During the last few days I've been trying to figure out how to achieve this and there is no way with my little knowledge.
I've designed three entities List <<-->> Item <<-->> Store in core data model designer. Each of them with only one attribute called "name".
The goal is to select a List, then show up all items within the list grouped by Store.
I've tried to use:
// Set entity.
entity = [NSEntityDescription entityForName:#"Item" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// Set object filters.
predicate = [NSPredicate predicateWithFormat:#"ANY list.name == %#", self.list.name];
[fetchRequest setPredicate:predicate];
// Set FRC
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:#"store.name" cacheName:nil];
Error:
'Invalid to many relationship in setPropertiesToFetch: (store.name)'
and this way too for populating rows by-hand (I don't know how yet):
// Set entity.
entity = [NSEntityDescription entityForName:#"Store" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// Set object filters.
predicate = [NSPredicate predicateWithFormat:#"SUBQUERY(item.list, $x, $x.name == %#).#count > 0", self.list.name];
[fetchRequest setPredicate:predicate];
// Set FRC
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:#"name" cacheName:nil];
Error:
'Only allowed one toMany/manyToMany relationship in subquery expression collection (SUBQUERY(item.list, $x, $x.name == "List02"))'
And also tried Fetched Properties and other ways that reach no-place.
Any ideas?
Regards.
Pedro.

Answer 1

Ok, here is the solution (danypata & Martin R give me the key).
To achieve this you should add a new entity in order to break the many-to-many relationship. The final Core Data model is: List <<-->> Item <-->> ItemStore <<--> Store. "ItemStore" entity doesn't need to have any attribute, just relationships.
The code...
- (NSFetchedResultsController *)fetchedResultsController
{
[...]
// Set entity.
entity = [NSEntityDescription entityForName:#"ItemStore" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
// Set object filters.
predicate = [NSPredicate predicateWithFormat:#"ANY item.list.name == %#", self.list.name];
[fetchRequest setPredicate:predicate];
// Edit the sort key as appropriate.
sortDescriptor = [[NSSortDescriptor alloc] initWithKey:#"item.name" ascending:YES];
sortDescriptors = #[sortDescriptor];
[fetchRequest setSortDescriptors:sortDescriptors];
// Set the batch size to a suitable number.
[fetchRequest setFetchBatchSize:20];
// Set FRC.
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:#"store.name" cacheName:nil];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
[...]
}
- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
Item *item;
ItemStore *itemStore;
[...]
itemStore = (ItemStore *)[self.fetchedResultsController objectAtIndexPath:indexPath];
item = itemStore.item;
cell.textLabel.text = item.name;
[...]
}
Final notes:
Break many-to-many relationship with a new entity (ItemStore) and two one-to-many relationships (Item <-->> ItemStore & ItemStore <<--> Store).
Due to Item's "store" relationship is optional (in my model), items without Store are not retrieved, if you want to do so you should assign them to a "No Store" object by default.
Thanks all.
Pedro.

Answer 2

As has been pointed out, you cannot fetch an items and group by store if one item can be in more than one store. However, using a NSFetchedResultsController for in display in a table view, or something similar, this is still possible (and quite usual).
Simply fetch the entity you want to group by, in this case Store. Adjust your table view datasource methods accordingly:
Number of sections:
return _fetchedResultsController.fetchedObjects.count
Title for section:
Store *store = _fetchedResultsController.fetchedObjects[indexPath.section];
return store.name;
Number of rows in section:
Store *store = _fetchedResultsController.fetchedObjects[indexPath.section];
return store.items.count;
One item in a store:
Store *store = _fetchedResultsController.fetchedObjects[indexPath.section];
Item *item = [[store.items sortedArrayUsingSortDescriptors:#[
[NSSortDescriptor sortDescriptorWithKey:#"sortAttribute" ascending:YES]]
objectAtIndex:indexPath.row];
// configure cell with information from item.
So while this is possible, perhaps you still want to rethink your data model. Does it really make sense that one item is in more than one Store? If it were not, you could use the plain vanilla fetched results controller.