NSFetchedResultController sort table view sections by multiple keys - ios

Question

Problem:
in my CoreData model, i have two entities called List and Item, they have one-to-many relationship. There are 2 types of List in my current design, system list and user list. System list is created by system during the very first launch and user list will be created by users anytime.
In my TVC, i would like to present all the items grouped by their associated lists(use list name as section title), in addition, i want all the system lists always placed on top of the user lists.
For instance, consider i have all the instances of list in DB as below:
System list: Inbox
User list: FruitList, ShoppingList
The expected result will be:
Inbox
InboxItem1
InboxItem2
FruitList
FruitListItem1
ShoppingList
Apple
Orange
The Approach:
With FRC, i can a achieve the first goal by setting sectionNameKeyPath as "list_name" to the FRC. But i have problem with fixing position the system list (Inbox in this case, it should always at index 0). However, the problem comes as user input their list with name start with "A" - "J" and all those lists will be placed on top of inbox, and i did try to fix it by adding "list_type" as secondary sort key, but it doesn't work.
NSFetchRequest *requst = [[NSFetchRequest alloc]initWithEntityName:ENTITY_ITEM];
NSArray *sortDescriptors = #[
[NSSortDescriptor sortDescriptorWithKey:#"belongsToList.list_name" ascending:YES],
[NSSortDescriptor sortDescriptorWithKey:#"belongsToList.list_type" ascending:NO]];
requst.sortDescriptors = sortDescriptors;
requst.predicate = [self predicateForListType:type selectedList:list];
requst.fetchBatchSize = 20;
_fetchedResultsController = [[NSFetchedResultsController alloc]initWithFetchRequest:requst managedObjectContext:self.managedObjectContext sectionNameKeyPath:#"belongsToList.list_name" cacheName:nil];
Temporary solution:
The temp solution i can think of to solve this problem is to add prefix string to the list name and store them as another attribute from DB and use it as sectionNameKeyPath.
e.g.
"0-" - for system lists ("Inbox" -> "0-Inbox")
"1-" - for user list ("ShoppingList" -> "1-ShoppingList")
and remove the prefix in -tableView:titleForHeaderInSection: . But this solution creates overhead and i think there should be much better solution out there since it is a common problem that all developers will face while they are dealing with FRC and TBV.
Any input??!!!

Answer 1

You can add a transient attribute like sectionIdentifier to the List entity and sort like this: use
[NSSortDescriptor sortDescriptorWithKey:#"list.sectionIdentifier" ascending:YES]
as the first sort descriptor.
The sectionIdentifier could be anything which sorts the way you intend ("System" first) and from which you can reconstruct the proper section name to return in titleForHeaderInSection.