Use Library in Model

Hello,
I'm creating a custom library, like this:

PHP Code:

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class my_class {
        protected $SDP;
        public function __construct()
        {
            $this->SDP =& get_instance();
            $this->SDP->load->database();
        }
        public function test4()
        {
            $query = $this->SDP->db->get('unit');
            return $query->result_array();
        } 


and create this Model:

PHP Code:

<?php
class my_model extends CI_Model {

        public function __construct()
        {
          $this->load->library('my_class');
        }
       public function test1()
        {
          $q = $this->my_class->test4();
          var_dump($q);
        } 


Is my code follow the MVC and Codeigniter's standards for use Library and Model?
If it's wrong please help me to fix it.