list menu and submenu from two table

Hi, I am new in codeigniter and I need help off course for one problem.

I have two tables, menu and submenu.

Table menu:
id_menu, menu_name
1, Menu 1
2, Menu 2
3, Menu 3

Table submenu;
id_sub, id_menu, sub_name
1,1, Sub 1.1
2,1, Sub 1.2
3,1, Sub 1.3
4,2, Sub 2.1
5,3, Sub 3.1
6,3, Sub 3.2

How I can using MVC get in view next list:

Menu 1
 Sub 1.1
 Sub 1.2
 Sub 1.3
Menu 2
 Sub 2.1
Menu 3
 Sub 3.1
 Sub 3.2

Thanks in advance