codeigniter get last insert id

$id = $this->db->insert_id();  $db = db_connect();
  $query = $db->query("SELECT * FROM users ORDER BY id DESC LIMIT 1");
  $result = $query->getRow();   $this->db->insert('posts', $post_data);
   $insert_id = $this->db->insert_id();
   return  $insert_id;