How to get last insert id after insert query inside of a transaction in CodeIgniter

I have an insert query (active record style) used to insert the form fields into a MySQL table. I want to get the last auto-incremented id for the insert operation as the return value of my query but I have some problems with it.

Inside the controller:

function add_post(){
    $post_data = array(
        'id'            => '',
        'user_id'   =>  '11330',
        'content'   =>  $this->input->post('poster_textarea'),
        'date_time' => date("Y-m-d H:i:s"),
        'status'        =>  '1'
    );
    return $this->blog_model->add_post($post_data);
}

And inside model:

function add_post($post_data){
    $this->db->trans_start();
    $this->db->insert('posts',$post_data);
    $this->db->trans_complete();
    return $this->db->insert_id();
}

I get nothing as the return of the add_post in model.

Try this

function add_post($post_data){
   $this->db->insert('posts', $post_data);
   $insert_id = $this->db->insert_id();

   return  $insert_id;
}

In case of multiple inserts you could use

$this->db->trans_start();
$this->db->trans_complete();