控制器上的Codeigniter未定义属性

i have controller to show database record on table

function gkategorimaterial()
{
    $this->load->model('m_kategorimaterial');
    $data['kirim'] = $this->m_kategorimaterial->read(); //error pointing on this line
    $data['title'] = 'QB Ambil Kategori Material / Jasa';

    $this->load->view('head',$data);
    $this->load->view('g/gkategorimaterial',$data);
}

that gkategorimaterial() was adapted from

function vkategorimaterial()
{
    $this->load->model('m_kategorimaterial');

    $data['kirim'] = $this->m_kategorimaterial->read();
    $data['title'] = 'QB Kategori Material';

    $this->load->view('head',$data);
    $this->load->view('v/vkategorimaterial',$data);
}

which has the some function, to show the table, the vkategorimaterial() success to retrieve the record.

here is the model of `m_kategorimaterial

function read()
{
    $query = $this->db->order_by('Kode_Kategori_Material_Jasa','DESC');
    $query = $this->db->get('ms_kategori_material');

    if($query->num_rows()>0)
    {     
        return $query->result();
    }
    else
    {
        return null;
    }
}

A PHP Error was encountered

Severity: Notice

Message: Undefined property: C_materialjasa::$m_kategorimaterial

Filename: controllers/c_materialjasa.php

Line Number: 140

i believe it has nothing to do with the models, but i don't know what the error is, since i just copy paste the first controller which is vkategorimaterial() controller, and rename the variable

Are database loaded on m_kategorimaterial?

$this->load->database(); 

or set in ./application/config/autoload.php

$autoload['libraries'] = array('database');  

Or try add parent in controller:

function __construct()
{
    parent::__construct();
}

Loading a model:

Controller:

$this->load->model('Model_name');

Model ('models/Model_name.php')

class Model_name extends CI_Model {

    function __construct()
    {
    parent::__construct();
    }

        function read() { return; }
}

Make sure that is model file CHMOD-ed to 0755!

my bad ! i wrote class classname extends CI_Controller on the model file