Sorting NSFetchedResultsController using a to
SCENARIO
I have two entities: Item and ListDetail (which contains prices for different lists for every item). This is absolutely needed and I can't provide a price attribute for the Item entity because every item can have more prices for different dynamic lists (retail, b2b ecc.).
The relationship is:
Item (lists) <------->> (item) ListDetail
The current active list in my app change dinamically, so let's say I have an integer variable with the current active list: _ACTIVE_LIST_CODE_. When I need a price for an item object I use an helper method on the Item class:
-(NSNumber*) getPrice {
NSSet *lists=[self.lists filteredSetUsingPredicate: [NSPredicate predicateWithFormat:@"listId == %d",_ACTIVE_LIST_CODE_]];
ListDetail *activeList=[[lists allObjects] objectAtIndex:0];
return activeList.price;
}
THE PROBLEM
I use a UITableView with NSFetchedResultController in order to select and show some items for different sections. Nothing special. I would like to order the fetchedObjects using the items price for the active list. If price was an attribute of Item I would added simply a sort descriptor to the fetch request like so:
[NSSortDescriptor sortDescriptorWithKey:@"price" ascending:YES];
But as said before this is not possible, price is a dynamic attribute.
If using transient properties was possible for sort descriptors, I would set a price transient properties calculated on fly using my helper method. Nothing to do.
Using a keypath in the descriptor like "lists.price" is not possible (or maybe I don't know how to do that), just because it's a to-many relationship and it's modeled with a NSSet.
I tried some workaround, without success:
1) observing _ACTIVE_LIST_CODE_ changes to set items price in a non-transient attribute.
2) after the fetch request, before presenting the table view, reorder a brand new array with fetched objects using the transient "price" property, iterate the orderdered array following an ascending integer index "i" and assigning this value to a non-transient property "order" for the Item entity. Using "order" for sort descriptor in the fetch request. (This approach is described here: Re-ordering NSFetchedResultsController)
Both of them works, but they slow down performance because I have thousands of items in the fetch results... Any idea?
How about fetching ListDetail instead? You could restrict and sort with the appropriate predicates and sort descriptors, exactly as you propose.
fetchRequest.predicate =
[NSPredicate predicateWithFormat:@"listID = %@", activeListCode];
fetchRequest.sortDescriptors =
@[[NSSortDescriptor sortDescriptorWithKey:@"price" ascending:YES]];
Now, to group by some attribute of item should be simple and efficient because it is a to-one relationship. Your fetched results controller's sectionNameKeyPath can be something like
@"item.category"