Get 500 his page isn’t working when run migrate codeigniter2 - codeigniter-2

Question

I get a 500 page isn’t working when run migrate codeigniter2. I am new here and it's take me days from this bug
Migate controller
<?php
class Migrate extends CI_Controller
{
public function index()
{
$this->load->library('migration');
if ($this->migration->current() === FALSE)
{
show_error($this->migration->error_string());
}
}
}
Config database
<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
$active_group = 'default';
$active_record = TRUE;
$db['default']['hostname'] = 'localhost';
$db['default']['username'] = 'root';
$db['default']['password'] = 'passw';
$db['default']['database'] = 'first_app';
$db['default']['dbdriver'] = 'mysql';
$db['default']['dbprefix'] = '';
$db['default']['pconnect'] = FALSE;
$db['default']['db_debug'] = FALSE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = '';
$db['default']['char_set'] = 'utf8';
$db['default']['dbcollat'] = 'utf8_general_ci';
$db['default']['swap_pre'] = '';
$db['default']['autoinit'] = TRUE;
$db['default']['stricton'] = FALSE;
Migration file
<?php
defined('BASEPATH') OR exit('No direct script access allowed');
class Migration_Add_blog extends CI_Migration {
public function up()
{
$this->dbforge->add_field(array(
'blog_id' => array(
'type' => 'INT',
'constraint' => 5,
'unsigned' => TRUE,
'auto_increment' => TRUE
),
'blog_title' => array(
'type' => 'VARCHAR',
'constraint' => '100',
),
'blog_description' => array(
'type' => 'TEXT',
'null' => TRUE,
),
));
$this->dbforge->add_key('blog_id', TRUE);
$this->dbforge->create_table('blog');
}
public function down()
{
$this->dbforge->drop_table('blog');
}
}
and when I tried to find buss using echo, the following line cannot run:
$this->load->library('migration');
I don't know why?
Please help me .