Error updating unique data in CodeIgniter - php

Question

I'm using CodeIgniter. I can't update data, because I make the database field is unique.
My controller:
function simpanadditional(){
$this->form_validation->set_rules('nama','Nama Additional','trim|required');
$this->form_validation->set_rules('harga','Harga Additional','required');
if($this->form_validation->run() == FALSE){
redirect('tambah-additional','refresh');
}else{
$cek = $this->my_model->ceknamadditional($this->input->post('nama'));
if(count($cek) > 0){
$this->session->set_flashdata('message','nama tidak boleh sama');
redirect('tambah-additional','refresh');
}else{
$this->my_model->simpanadditional();
redirect('admin/tabel_additional','refresh');
}
}
}
function editadditional($id){
if($this->session->userdata('logged_in'))
{
$session_data = $this->session->userdata('logged_in');
$data['username'] = $session_data['username'];
$data['title'] = 'Edit Additional';
$data['main'] = 'admin/additional/form_edit_additional';
$data ['getdata'] = $this->my_model->editadditional($id);
$data['getgroup'] = $this->my_model->editgroup($session_data['username']);
$this->load->view('admin/index',$data);
} else {
redirect('admin/login', 'refresh');
}
}
My model:
function simpanadditional(){
$data = array(
'nama_additional' => $this->input->post('nama'),
'harga_additional' => $this->input->post('harga'),
);
if($this->input->post('id')){
// ini untuk update nya
$this->db->where('id_additional',$id);
$this->db->update('additional',$data);
}else{
// ini untuk insert nya
$this->db->insert('additional',$data);
}
}
function editadditional($id){
$data = array();
$this->db->select('*');
$this->db->from('additional');
$this->db->where('id_additional',$id);
$hasil = $this->db->get();
if($hasil->num_rows() > 0){
$data = $hasil->result();
}
$hasil->free_result();
return $data;
}
This is views of form additional:
<div class="item form-group">
<label class="control-label col-md-3 col-sm-3 col-xs-12" for="name">Nama <span class="required">*</span>
</label>
<div class="col-md-6 col-sm-6 col-xs-12">
<input id="name" class="form-control col-md-7 col-xs-12" data-validate-length-range="6" data-validate-words="2" name="nama" value="<?php echo $data->nama_additional ?>" type="text">
<input type="hidden" name="id" value="<?php echo $data->id_additional ?>">
</div>
</div>
<div class="item form-group">
<label class="control-label col-md-3 col-sm-3 col-xs-12" for="name">Harga <span class="required">*</span>
</label>
<div class="col-md-6 col-sm-6 col-xs-12">
<input id="icon" class="form-control col-md-7 col-xs-12" data-validate-length-range="6" data-validate-words="2"
name="harga" value="<?php echo $data->harga_additional ?>" type="text" onkeypress="return isNumberKey(event)">
</div>
</div>
Can anyone help?