how to get last insert id after insert query in codeigniter active record

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();

From the documentation:

$this->db->insert_id()

The insert ID number when performing database inserts.

Therefore, you could use something like this:

$lastid = $this->db->insert_id();

$id = $this->db->insert_id();

A transaction isn't needed here, this should suffice:

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