extend DB table with edit by field

hello,

we have same backend make by a freelancer and he end the job.
now we try to understand codeigniter and this doing.  

we want to extend our database model with a hidden fielt edit_by with value from the editing user logined right now in the backend.
DB Field names "edit_by"

any help for us???

CONTROLLER

PHP Code:

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

class Bars extends CI_Controller {

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

 $this->load->database();
 $this->load->helper('url');

 $this->load->library('grocery_CRUD');
 }

 public function data_output($output = null)
 {
 $data['page'] = 'barsview';
 $data['output']=$output;
      $this->load->view('template', $data);
 }
 function modified_callback($post_array, $primary_key) {
 $post_array['modified']=time();
 return $post_array;
 }
 function inserted_callback($post_array, $primary_key) {
 $post_array['modified']=time();
 return $post_array;
 }
 public function index()
 {
 try{
 $crud = new grocery_CRUD();

 //$crud->set_theme('datatables');
 $crud->set_table('bars');
 $crud->set_subject('Bar');
 $crud->change_field_type('show_contact_details','true_false');
 $crud->change_field_type('modified','invisible');
 $crud->callback_before_update(array($this,'modified_callback'));
 $crud->callback_before_insert(array($this,'inserted_callback'));
 $crud->set_rules('popularity','Popularity','integer');
 $crud->set_rules('popularity', 'Popularity', 'required|xss_clean|max_length[5]');
 $crud->required_fields('name', 'description', 'image', 'address', 'zipcode', 'city', 'latitude', 'longitude', 'availability', 'price', 'priority', 'reviews');
 $crud->unset_texteditor('reviewlink','full_text');
 $crud->unset_texteditor('moreinfo','full_text');
 $crud->unset_texteditor('overview','full_text');
 $crud->unset_texteditor('address','full_text');
 $crud->set_field_upload('image','assets/restaurantsUploads/');
      $crud->callback_column('image',array($this,'_callback_fun'));
 $crud->columns('name', 'description', 'image', 'address', 'zipcode', 'city' , 'popularity', 'edit_by');
 
 $output = $crud->render();

 $this->data_output($output);

 }catch(Exception $e){
 show_error($e->getMessage().' --- '.$e->getTraceAsString());
 }
 }

 
} 


VIEW

PHP Code:

<?php 
foreach($css_files as $file): ?>
 <link type="text/css" rel="stylesheet" href="<?php echo $file; ?>" />
<?php endforeach; ?>
<?php foreach($js_files as $file): ?>
 <script src="<?php echo $file; ?>"></script>
<?php endforeach; ?>

    <div>
 <?php echo $output; ?>
    </div>