关系codeigniter，codeigniter-base-model

:)

I have a problem with relationships in codeigniter , I'm doing a small blog.

the problem is that I need in the post by the user ID called the USERNAME that created the post .

but I can not do it step my code , I hope you can help me

controller

  public function show($post = NULL)
{
  $this->load->model('post_model', 'posts_model');
  $this->load->helper('url');
  $postid = $this->uri->segment(4);
  $post = $this->posts_model->get_by('id', $postid);
  $this->data = array(
      'title' => 'Posts',
      'subtitle' => 'Post',
      'navClass' => 'tofront blue-bar',
      'subnavClass' => 'toback blue-bar',
      'isfab' => true,
      'message' => $this->session->flashdata('message'),
      'posts_call' => $post,
      'current' => current_url(),
      'sidebar' => true,
  );

  $this->session->set_userdata('last_page', current_url());
  $this->theme->render('post/show', $this->data);
}

model

class Post_model extends SP_Model
{
  public $belongs_to = array( 'post' => array( 'primary_key' => 'post_author' )  );
}

DB:

Foreign key post_author

need relation with id

thanks in advance! :D