如何在swift中对父和子CoreData元素进行排序

我的CoreData模型中有3个实体。类别、子类别、库存。连接到子类别的库存和使用关系连接到类别的子类别。所以,当我想要获得库存和子类别时,我会使用类别数据。所以我的问题是我想用字母顺序对所有三个实体数据进行排序。

class func getAllCategoriesObject() -> [DBCategories]?
    {
        let context = getContext()

        let fetchRequest:NSFetchRequest<DBCategories> = DBCategories.fetchRequest()
        fetchRequest.returnsObjectsAsFaults = false

        do{
            let fetchedData = try context.fetch(fetchRequest).sorted(by: { (objCategory1, objCategory2) -> Bool in
                (objCategory1.subcategories?.allObjects as? [DBSubCategories] ?? []).sorted { (objSubCategory1, objSubCategory2) -> Bool in

                    (objSubCategory1.inventories?.allObjects as? [DBInventories] ?? []).sorted { (objInventory1, objInventory2) -> Bool in

                        return objInventory1.title?.capitalized ?? "" < objInventory2.title?.capitalized ?? ""
                    }

                    return objSubCategory1.title?.capitalized ?? "" < objSubCategory2.title?.capitalized ?? ""
                }

                return objCategory1.title?.capitalized ?? "" < objCategory2.title?.capitalized ?? ""
            })



            return fetchedData
        }
        catch
        {
            print( error.localizedDescription)
        }
        return []
    }

我尝试了上面的代码,但没有得到输出。