Error : Message: Undefined property: my_controller::$session

[eluser]Sathishkumar[/eluser]
Guys,
I'm getting following error. Please help me to resolve this.

Code:

A PHP Error was encountered

Severity: Notice

Message: Undefined property: Login::$session

Filename: core/Model.php

Line Number: 50

My Code :

Code:

$query = $this->db->get_where("user", array('username' => $username,'password' => md5($password),'is_active' => 0, 'is_delete' => 0));
  
    foreach($query->result_array() as $value)
       $session_data = array(
       'user_id' => $value['user_id'],
       'username'  => $username,
       'role_id' => $value['role_id'],
       'logged_in'  => TRUE                
    );
  
   $this->session->sess_destroy();
   $this->session->set_userdata($session_data);