$this数据库更新工作，但数据库不更改

我目前正在开发一个使用 php jquery 和 bootstrap 的程序，更新功能，有一个错误，使数据库不会更改但脚本正常运行，也许有人能够帮助我我的错误在哪里？ 谢谢

我的控制器

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
class Subbidang extends CI_Controller {
public function index()
{
    $this->load->model('model_subbidang');
    $this->model_security->getsecurity();
    $isi['content'] = 'Subbidang/tampilan_datasubbidang';
    $isi['judul'] = 'Master';
    $isi['sub_judul'] = 'Sub Bidang'; 
    $isi['data'] = $this->db->get('tsubbidang');
    $this->load->view('tampilan_home',$isi);
}
public function tambah()
{
    $this->load->model('model_subbidang');
    $this->model_security->getsecurity();
    $isi['content'] = 'Subbidang/form_tambahsubbidang';
    $isi['judul'] = 'Master';
    $isi['sub_judul'] = 'Sub Bidang'; 
    $this->load->view('tampilan_home',$isi);
}
public function simpan()
{
    $this->model_security->getsecurity();
    $this->load->model('model_subbidang');
    $key['IDSubBidang'] = $this->input->post('kode');
    $data['IDSubBidang']    = $this->input->post('kode');
    $data['IDBidang']       = $this->input->post('bidang');
    $data['NamaSubBidang']  = $this->input->post('subbidang');
    $query = $this->db->get_where('tsubbidang',$key);
    if ($query->num_rows()>0)
    {
        $this->db->update('tsubbidang',$key,$data);
        //echo "Data Berhasil Diubah";
        echo "Update";
    }
    else
    {
        $this->db->insert('tsubbidang',$data); 
        //xecho "Data Berhasil Disimpan";
        echo 'Insert';
    }
}
} 
/* End of file welcome.php */
/* Location: ./application/controllers/welcome.php */

我的观点

<script type="text/javascript">
$(document).ready(function(){

$("#simpan").click(function(){
var string = $("#my-form").serialize();
//alert(string);
$.ajax({
            type    : 'POST',   
            url     : '<?php echo site_url();?>/subbidang/simpan',
            data    : string,
            success : function(data){
                alert(data); 
            } 
        });
    });
});
</script>
<form class="form-horizontal" name="my-form" id="my-form">
<div class="control-group">
    <label class="control-label"bidang</label>
    <div class="controls">
        <select name="bidang" id="bidang">
            <option value="">-Pilih-</option>
            <?php
            $bidang = $this->db->get('tbidang');
            foreach ($bidang->result() as $row) {
            ?>
            <option value="<?php echo $row->IDBidang;?>"><?php echo $row->Bidang;?> </option>
            <?php };?>
        </select>
    </div>
</div>
<div class="control-group">
    <label class="control-label">Kode Sub Bidang</label>
    <div class="controls">
        <input type="text" name="kode" id="kode" palceholder="kode" class="span3">
    </div>
</div>
<div class="control-group">
    <label class="control-label">Sub Bidang</label>
    <div class="controls">
        <input type="text" name="subbidang" id="subbidang" palceholder="Sub Bidang" class="span3">
    </div>
</div>
<div class="controls">
    <button type="button" name="simpan" id="simpan" class="btn btn-primary btn-small"> Simpan</button>
</div>

我的模型

    <?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
class Model_subbidang extends CI_model {
    public function gettampildatasubbidang($key)
    {
        $this->db->where('IDBidang',$key);
        $query = $this->db->get('tbidang');
        if($query->num_rows()>0)
        {
            foreach ($query -> result() as $row) {
                $hasil = $row->Bidang;
            }
        }
        else
        {
            $hasi='';   
        }
        return $hasil;
    }
    public function getlistsubbidang()
    {
        return $this->db->get('tbidang');
    }
    public function getdata($key)
    {
        $this->db->where('IDBidang',$key);
        $hasil = $this->db->get('tBidang');
        return $hasil;
    }
    public function getupdate($key,$data)
    {
        $this->db->where('IDBidang',$key);
        $this->db->update('tBidang',$data);
    }
    public function getinsert($key,$data)
    {
        $this->db->insert('tBidang',$data); 
    }A

}
/* End of file welcome.php */
/* Location: ./application/controllers/welcome.php */

回显工作正常，但数据库不会更改。插入功能也可以正常工作