在codeigniter中处理ajax时会话消失

Hi all currently am working in chat functionality in my project.

When a user start his conversation an id will be saved in the session, the id will same throughout the chat id value stored in session gets disappear

In the following code i have mentioned the issues

Initially there is no session value for id then it will enter into to the if condition and session value will be created for id during next loop it should not allow to the if condition but in my case its allowing its because the session value get disappear

extract($this->input->post());
  $loc=get_location($this->session->userdata('ip_address'));
  $this->session->set_userdata('chatusername',$username);
  $this->session->set_userdata('chatemail',$email);

   if($this->session->userdata('id')=='')
      {

         $messageid= sprintf("%06d", mt_rand(100000, 999999));
         $this->session->set_userdata('id',$messageid);

      }
    $data['message_id']=$this->session->userdata('id');
    $data['username']=$username;
    $data['email']=$email;
    $data['department_id']=$department_id; 
    $data['message']=$message;
    $this->db->insert('message',$data);

help me to solve this issue am confused can't find the cause of the issue