How to use set_model function ?

Dear all

I'm newbies to CodeIgniter and Grocery CRUD.
Thanks for GroceryCRUD's author, John Skoumbourdis.
This project make my life more easy and more relax

I wanna use set_model function which handle some work

set_model is used in my code (Controller's method):



$crud->set_model('My_model');

and then, created my_model.php file in application/model folder



class My_model extends grocery_CRUD_Model  {

function list_all(){

  $this->db->order_by('sn','desc');

  return $this->db->get("table1");

}

}

But now, how can I use My_model's method list_all in Grocery CRUD code ?
Because I used the code:



$this->My_model->list_all();

Nothing happen

and used this code:



$crud = new grocery_CRUD();

$crud->My_model->list_all();

Show me error message:
Message: Undefined property: grocery_CRUD::$My_model

Today, I searched all day in this topic, but I can't found any about this topic
Thanks for anybody who can help me

And my English text is a little weak, please don't mind.

hogogo

fdias