callback on code igniter always return false - php

Question

i am currently making this part that can create something to input to database. i used callback function of code igniter to check availability of some item code of a non auto increment table.
i always get the message of the callback '{field} exists.' how do i fix this?
controllers
// CREATE /////////////////////////////////////////////////////////
public function create(){
$this->load->library('form_validation');
$this->form_validation->set_rules('JOB_CODE','Job Code','trim|required|min_length[2]|max_length[5]|callback_check_if_exists');
$this->form_validation->set_rules('JOB_NAME','Job Name','trim|required|max_length[30]');
if($this->form_validation->run() == FALSE){
$this->add_view();
}else{
$input = array(
'JOB_CODE' => $this->input->post('JOB_CODE'),
'JOB_NAME' => $this->input->post('JOB_NAME')
);
$this->Job_Titles_Model->insert($input);
}
}
/////////// FOR TABLES WITH NO AUTO INREMENT
public function check_if_exists($jobcode){
$this->load->model('Job_Titles_Model');
$availability = $this->Job_Titles_Model->check_if_exists($jobcode);
if($availability){
return TRUE;
}else{
return FALSE;
}
}
models
///// CREATE /////////////////////////////////////////////////////////
public function insert($input){
$insert = $this->db->insert('job_titles',$input);
}
/////////// FOR TABLES WITH NO AUTO INREMENT
public function check_if_exists($jobcode){ //CHECK IF JOBODE IS AVAILABLE
$sql = ('SELECT * FROM job_titles WHERE JOB_CODE = ?');
$data = array('JOB_CODE' => $this->input->post('JOB_CODE'));
if($result->num_rows() == 0){
return TRUE;
}else{
return FALSE;
}
}