CoreData how to add items to a NSSet which is related to an entity

In my project I created two entities: Container and Items. There's a one to many relationship between the two. In Container I called the relationship item and I set it to "To Many" because I want to have many items in each container. In Items I called the relationship container and I set it to "To One" because each item must be associated to one and only one container.

This aside, in my project I have a tableview where I can add new containers and when I press on a cell I can see its items. The code to create the containers is fine. The problem comes when I try to create the items: they are created successfully but they appear in each container.

In the following part I'll post the objects, how I save containers and how I save items:

• Objects:

extension Container {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Container> {
        return NSFetchRequest<Container>(entityName: "Container")
    }

    @NSManaged public var name: String?
    @NSManaged public var index: Int64
    @NSManaged public var items: NSSet?

}

extension Items {

    @nonobjc public class func fetchRequest() -> NSFetchRequest<Items> {
        return NSFetchRequest<Items>(entityName: "Items")
    }

    @NSManaged public var text: String?
    @NSManaged public var image: UIImage?
    @NSManaged public var index: Int64
    @NSManaged public var container: Container?

}

• Creating container:

let containerEntity = NSEntityDescription.entity(forEntityName: "Container", in: managedContext)!
        
        let items: NSSet = []
        
        let container = NSManagedObject(entity: containerEntity, insertInto: managedContext) as! Container
        
        ///Count is just an int variable
        container.setValue("Name", forKeyPath: "name")
        container.setValue(count, forKeyPath: "index")
        container.setValue(items, forKey: "items")
        
        do {
            try managedContext.save()

        } catch let error as NSError {
            print("Could not save. \(error), \(error.userInfo)")
        }

• Creating item:

  let itemEntity = NSEntityDescription.entity(forEntityName: "Items", in: managedContext)!
        
        let items = NSManagedObject(entity: itemEntity, insertInto: managedContext) as! Items
 
        items.setValue("item", forKeyPath: "text")
        items.setValue(count, forKeyPath: "index")
     
        
        do {
            try managedContext.save()

        } catch let error as NSError {
            print("Could not save. \(error), \(error.userInfo)")
        }
        

Anyway I think that instead of creating items entities I should edit the container entity adding to it items. To do that I tried this but I feel like I'm missing something because this thing crashes.

let fetchRequest:NSFetchRequest<NSFetchRequestResult> = NSFetchRequest.init(entityName: "container")
       
       do {
           let test = try managedContext.fetch(fetchRequest)
           
           let objectUpdate = test[containerIndex] as! NSManagedObject
           
           objectUpdate.setValue("item", forKey: "text")
           objectUpdate.setValue(count, forKey: "index")
           
           do{
               try managedContext.save()
           } catch {
               print(error)
           }
          
       }
       catch {
           print(error)
       }

So how can I add items to a specific container?

Plus, is it ok if two of my entities attributes are called index?

If set up correctly a there should be an addToItems()

anyContainerObject.addToItems(value: Items)