Query Builder Where Not Working With Join

I am working on my profile display library

And I need to be able to have a db->where for my user_permission table.

It works fine and displays correct data with out it but would like to find out why when ever I use this line below it does not work

PHP Code:

$this->CI->db->where('up.user_permission_id', 'u.user_permission_id'); 


But if I comment that line out I can get data.

How could I still use that line and display data.

PHP Code:

<?php

class Profile {

    public function __construct() {
        $this->CI =& get_instance();
        $this->CI->load->library('session');
    }

    public function user_info() {
        $this->CI->db->select('*, up.name as user_permission');
        $this->CI->db->from($this->CI->db->dbprefix . 'user_permission up', 'LEFT');
        $this->CI->db->join($this->CI->db->dbprefix . 'user u', 'u.user_permission_id = up.user_permission_id', 'LEFT');
        // Where user_permission table
        //$this->CI->db->where('up.user_permission_id', 'u.user_permission_id');
        // Where for user table
        $this->CI->db->where('u.user_id', (int)$this->CI->session->userdata('user_id'));
        
        $query = $this->CI->db->get();
        return $query->row_array();
    }
} 


Could you please explain why it return in no information with this $this->CI->db->where('up.user_permission_id', 'u.user_permission_id');
But when I comment it out it works? I am confused.

There's only one rule - please don't tell anyone to go and read the manual.  Sometimes the manual just SUCKS!