Creating submenu in MVC

You can make a few changes:

1. Add attitional attibute into MenuItemViewModel to store Children:

   // children
   public IEnumerable<MenuItemViewModel> Children { get; set; }
   

2. Add a new method GetMenuChildren() to generate a tree of MenuItemViewModel items instead of list.

   private IEnumerable<MenuItemViewModel> GetMenuChildren(IEnumerable<MenuItem> menuItems, int itemParentId, MultiDocumentQuery pages, int menuDepth = 0, int menuLevel = 0)
   {
       var items = menuItems
           .Where(x => x.NodeParentID == itemParentId)
           .Select(item => new MenuItemViewModel()
           {
               Children = GetMenuChildren(menuItems, item.NodeID, pages, menuDepth, menuLevel + 1),
               MenuItemText = item.MenuItemText,
               // Gets the URL for the page whose GUID matches the given menu item's selected page
               MenuItemRelativeUrl = pages.FirstOrDefault(page => page.NodeGUID == item.MenuItemPage).RelativeURL
           });
   
       return items;
   }
   

3. Call GetMenuChildren() method from GetMenu() method to get a menu model:

   public virtual ActionResult GetMenu()
   {
       // Loads all menu items using the page type's generated provider
       // Uses the menu item order from the content tree in the Kentico 'Pages' application
       var menuItems = MenuItemProvider.GetMenuItems()
           .Columns("NodeID", "NodeParentID", "MenuItemText", "MenuItemPage")
           .OrderBy("NodeOrder")
           .ToList();
   
       // Loads the pages selected within the menu items
       // The data only contains values of the NodeGUID identifier column
       var pages = DocumentHelper.GetDocuments()
           .WhereIn("NodeGUID", menuItems.Select(item => item.MenuItemPage).ToList())
           .Columns("NodeGUID");
   
       // Get Navigation Menu Root Item - should be the one
       var menuItemsRoot = MenuItemsProvider.GetMenuItems().First();
   
       // Creates a collection of view models based on the menu item and page data
       var model = GetMenuChildren(menuItems, menuItemsRoot.NodeID, pages, 0);
   
       return PartialView(MVCConsts.Menu.Views.ViewNames._SiteMenu, model);
   }
   

Note: MenuItemsProvider (additional MenuItems Page Type added) is required to get the root item id.