Message: mysql_real_escape_string() expects parameter 2 to be resource, boolean given Filename: mysql/mysql_driver.php Line Number: 346

I am using the following code to select from a MySQL database with a Code Igniter webapp:

My Model

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
class m_login extends CI_Model{
function __construct(){
    parent::__construct();
}

public function validate(){
    // grab user input
    $username = $this->security->xss_clean($this->input->post('username'));
    $password = $this->security->xss_clean($this->input->post('password'));

    $password = md5($password);
    // Run the query
    //$array = array('nip' => $this->db->escape_str($username), 'password' => $this->db->escape_str($password));

    //$this->db->select('pegawai.NIP, akun.PASSWORD, akun.ROLE');
    //$this->db->from('pegawai');
    //$this->db->join('akun', 'akun.ID_PEGAWAI = pegawai.ID_PEGAWAI');
    //$this->db->get_where('nip', $username);
    //$this->db->where($array);
    //$this->db->where('PASSWORD', 'd93591bdf7860e1e4ee2fca799911215');
    //$query = $this->db->get();        

    $sql = "SELECT NIP, PASSWORD, ROLE FROM pegawai LEFT JOIN akun ON pegawai.ID_PEGAWAI=akun.ID_PEGAWAI WHERE pegawai.NIP=('".$this->db->escape_str($username)."') AND akun.PASSWORD=('".$this->db->escape_str($password)."')";
    $this->db->query($sql);
    //$query = $this->db->query("SELECT NIP, PASSWORD, ROLE FROM pegawai LEFT JOIN akun ON pegawai.ID_PEGAWAI=akun.ID_PEGAWAI WHERE pegawai.NIP='$username' AND akun.PASSWORD='$password'");
    // Let's check if there are any results
    //var_dump($this->db);
    //var_dump($username);
    if($query->num_rows == 1)
    {
         //If there is a user, then create session data
        $row = $query->row();
        $data = array(
                'role' => $row->role,
                'nip' => $row->nip,
                'validated' => true
                );
        $this->session->set_userdata($data);
        return true;
    }
     //If the previous process did not validate
     //then return false.
    return false;
}
}
?>

I have error like this

A PHP Error was encountered 
Severity: Warning
Message: mysql_real_escape_string() expects parameter 2 to be resource, boolean given
Filename: mysql/mysql_driver.php
Line Number: 346
Backtrace:
File: C:\xampp\htdocs\simpeg\application\models\m_login.php
Line: 27
Function: escape_str
File: C:\xampp\htdocs\simpeg\application\controllers\login.php
Line: 26
Function: validate
File: C:\xampp\htdocs\simpeg\index.php
Line: 292
Function: require_once

Most likely, this is a syntax error, but I just can't figure out which part of my code is responsible.

I've also gone through the Queries section of the user guide of CodeIgniter, but it wasn't explained clearly there.

Can anyone please tell me where my mistake is, and what is the correct syntax for what I'm trying to do?