Solusi Untuk Saya Loginnya Gagal

saya membuat aplikasi login dengan CI+jQueri Mobile. masalahnya saat saya login menggunakan password dan username yang benar aplikasi tidak menampilkan halaman home admin tpi menampilkan halaman login kembali apakah sessionnya tidak terbentuk??
saya masih pemula CI tolong solusinya??

berikut souce codenya :

controllers/app.php

PHP Code:

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
class App extends CI_Controller {
      
      function __construct(){
        parent::__construct();
        session_start();
        $this->load->helper('url');
        $this->load->model('MAdmins','',TRUE);
        
    }

    public function index()
    {
        $session=isset($_SESSION['username']) ? $_SESSION['username']:'';
        if($session=="")
           {
            $this->login();
        }
        else 
        {
              $this->homeadmin();
        }
    }
    
    public function login()
    {
        $session=isset($_SESSION['username']) ? $_SESSION['username']:'';
        if($session==""){
        $this->load->view('admin/login_admin');
        //redirect(site_url('admin/login_admin'));
        }
    }
    
    public function verifylogin()
    {
        //Aksi untuk melakukan validasi
        $this->load->library('form_validation');
        $this->form_validation->set_rules('username', 'Username', 'trim|required|xss_clean');
        $this->form_validation->set_rules('password', 'Password', 'trim|required|xss_clean|callback_check_database');
        if($this->form_validation->run() == FALSE)
        {
        //Jika validasi gagal user akan diarahkan kembali ke halaman login
        $this->login();
        //redirect(site_url('app/login'));
        }
        else
        {
        //Jika berhasil user akan di arahkan ke private area 
        //$_SESSION['username']=$session_username;
        $this->homeadmin();
        //redirect(site_url('app/homeadmin'));
        }
    }
    
    function check_database($password)
    {
    //validase field terhadap database 
    $username = $this->input->post('username');
    
    //query ke database
    $result = $this->MAdmins->login($username, $password);
    
        if($result)
        {
        $sess_array = array();
        foreach($result as $row)
        {
            $sess_array = array(
            'id' => $row->id,
            'username' => $row->username
            );
            $this->session->set_userdata('logged_in', $sess_array);
            $_SESSION['username']=$session_username;
        }
        return TRUE;
        }
        else
        {
        $this->form_validation->set_message('check_database', 'Invalid username or password');
        return false;
        }
    }
    
    public function homeadmin()
    {
        $session=isset($_SESSION['username']) ? $_SESSION['username']:'';
        if($session=="")
        {
        //Jika tidak ada session di kembalikan ke halaman login
        
        $this->login();
        }
        else
        {
        //$pecah=explode("|",$session);
        //$data["nama"] = $pecah[1];
        $this->adminhome();
        //redirect(site_url('admin/v_home/admin'));
        }
    }
        
    public function logout() {
        $this->session->unset_userdata('username');
        $this->session->unset_userdata('status');
        session_destroy();
        $this->homeadmin();
    }
    

} 


models/madmins.php

PHP Code:

<?php
class MAdmins extends CI_Model
{
function __construct()
{
parent::__construct();
}
//fungsiceklogin
function login($username, $password)
{
        $this -> db -> select('id_admin, username, password');
        $this -> db -> from('tb_admin');
        $this -> db -> where('username = ' . "'" . $username . "'"); 
        $this -> db -> where('password = ' . "'" . MD5($password) . "'"); 
        $this -> db -> limit(1);

        $query = $this -> db -> get();

        if($query -> num_rows() == 1)
        {
            return $query->result();
        }
        else
        {
            return false;
        }
}
}
?>

views/admin/login_admin.php

PHP Code:

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" >
<title>Login</title>
    <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no" />
    <link href="<?php echo base_url(); ?>/assets/jquery.mobile-1.4.4.min.css" rel="stylesheet" type="text/css" />
    <script type="text/javascript" src="<?php echo base_url(); ?>/assets/jquery.min.js"></script>
    <script type="text/javascript" src="<?php echo base_url(); ?>/assets/jquery.mobile-1.4.4.min.js"></script>
</head>
<body>
<div data-role="page" id="indexPage" data-theme="a">
<div data-role="header" data-position="fixed" data-theme="b">
<h1>Login Administrator</h1>
</div>
<div role="main" class="ui-content">
    <?php echo validation_errors(); ?>
    <form method="post" action="<?php echo site_url('app/verifylogin');?>">
            <div class="ui-field-contain" id="form">
            <label for="username">Username :</label>
            <input type="text" name="username" id="text1" value="" placeholder="Username"/>
            </div>
            
            <div class="ui-field-contain" id="form">
            <label for="password">Password :</label>
            <input type="password" name="password" id="text2" value="" placeholder="Password"/>
            </div>
            <!--tombol untuk login dan mengosongkan text  data-theme="b" data-icon="delete"-->
            <br/>
            <input type="submit" value="Login" data-theme="b" data-icon="check"/>
            <input type="reset" value="Reset" data-theme="b" data-icon="delete"/>
    </form>
</div>
<div data-role="footer" data-position="fixed" data-theme="b">
<h1>Yabin Gilroi Kristanto</h1>
</div>
</div>
</body>
</html> 


views/admin/v_home_admin.php

PHP Code:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
       <link href="<?php echo base_url(); ?>/assets/jquery.mobile-1.4.4.min.css" rel="stylesheet" type="text/css" />
        <script type="text/javascript" src="<?php echo base_url(); ?>/assets/jquery.min.js"></script>
        <script type="text/javascript" src="<?php echo base_url(); ?>/assets/jquery.mobile-1.4.4.min.js"></script>
    
    <title>Home</title>
</head>
<body>
<!-- menu panel -->
<div data-role="page" id="indexPage" data-theme="a">
    
        <div data-role="header" data-position="fixed" data-theme="b">
        <h3>Aplikasi LBS-Toko Furniture</h3>
         <a href="" data-icon="bars">Log-Out</a>
        </div>

            <div role="main" class="ui-content">
                   <ul data-role="listview">
                        <br/>
                        <li><a href="#">Pengolahan Data Admin</a></li>
                        <li><a href="#">Pengolahan Data Toko Furniture</a></li>
                        <li><a href="#">Pengolahan Data Buku Tamu</a></li>
                    </ul>             
         
             </div>   
        <div data-role="footer" data-position="fixed" data-theme="b">
        <h1>@ 2015-Yabin Gilroi Kristanto</h1>
        <a href="<?php echo site_url('app/login');?>" data-ajax="false" class="ui-btn ui-icon-lock ui-btn-icon-left ui-btn-right">Kembali</a>
        <!--<p><center><br><h6>Page rendered in {elapsed_time} seconds</h6></br></center></p> -->
        </div>
</div>
</body>
</html> 


$_SESSION['username'] ny di isi dimn gan? kalau menurut coding agan $_SESSION['username']=='' semua.