I have a MySQL query which is this:

```
SELECT * FROM tbl_post WHERE tbl_post.post_id NOT IN 
(SELECT tbl_readsave.post_id FROM tbl_readsave)

```

I want to convert it into Codeigniter Active records, so I used the following code segment:

```
    $this->db->select('tbl_readsave.post_id');
    $queryReadSave = $this->db->get('readsave');
    $this->db->where_not_in('post_id', $queryReadSave->result_array());
    $queryNewPost = $this->db->get('readsave');
    if($queryNewPost->num_rows()>0)
    {
        return $queryNewPost->result_array();
    }
    else
        return false;

```

However, the code throws me an error, which is like the following:-

```
Error Number: 1054

Unknown column 'Array' in 'where clause'

SELECT * FROM (`tbl_readsave`) WHERE `post_id` NOT IN (Array)

Filename: /var/www/html/teamF/tharjumal/models/webservice_model.php

Line Number: 28

```

How can I convert the above stated query into Codeigniter Active Records format?