不在codeigniter中工作的模型

Using CI_Version 2.2.1 Models not connected in controller. I'm getting an error like

Undefined property: Administrator::$administrator_model & Fatal error: Call to a member function admin_authentication() on a non-object ...

class Administrator extends CI_Controller {

 public function _construct()
 {
    parent::__construct();  
    $this->load->model('administrator_model');
   }

public function login_authentication()
{
  $username=$this->input->post('username');
    $password=$this->input->post('password');
  $check_login = $this->administrator_model->admin_authentication($username,$password);

}

Models

class Administrator_model extends CI_Model {

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


public function admin_authentication($username,$password)
{
   $this->db->select('*');
    $this->db->from('user_credential');
    $this->db->where('db_username',$username);
    $this->db->where('db_password',$password);
    $query = $this->db->get();
    return $query->result_array();

}

}

Please help to solve this issue, Thanks in advance.

class Administrator_model extends CI_Model {
function __construct()
{
    parent::__construct();  
    $this->db = $this->load->database('default', true);
}

You have to load the db library first. OR In

autoload.php :

$autoload[‘libraries’] = array(‘database’);

is your model file in models folder root? eg: application/models/administrator_model.php

or it is in a sub folder eg: application/models/admin/administrator_model.php then you have to load like $this->load->model('admin/administrator_model')

and the file name should be Ucfirst in CI3.0 but i forgot if it is necessary for CI2.2. (if you are using windows the filename wont bother until you deploy it to a linux server :)