Ajax错误响应-codeigniter-html而不是json

当我进入网络查看我的响应时，我总是得到未找到的页面，从未得到json响应。 在ChromeDevTool的响应中，我从整个加载页面获取html。 我已经在一个空白的项目中尝试了我的代码，它的工作没有问题。不知道我错过了什么。 在我的控制器的类别\u id中没有任何值

<script type="text/javascript">
    $(document).ready(function(){
        $('#category').change(function(){ 
            var id=$(this).val();
            var cat_selected = $('#category :selected').text();
            var url = 'https://www.test.com/course/';
           console.log(id);
            $.ajax({
                url : "<?php echo site_url('product/get_sub_category');?>",
                method : "POST",
                data : {id: id},
                async : true,
                dataType : 'json',
                success: function(data){
                    var html = '<option value="">No Selected</option>';
                    var i;

                    for(i=0; i<data.length; i++){
                        html += '<option id='+data[i].id+' value='+ url + data[i].url+'>'+data[i].display_name+'</option>';
                    }
                    var sub_cat = ($('#sub_category').val());
                    console.log($('#category :selected').text());
                   console.log(sub_cat);
                    $('#sub_category').html(html);
                    $('#sub_category').change(function(){ 
                   if(sub_cat == 'Not Selected') {
                        console.log($('#category :selected').val());
                   } else {
                   }
                   
                   });
                    $('#sub_category').click(function(){ 
                    var selected = $(this).find('option:selected').val();
                    $('#submit').click(function(e){
                        e.preventDefault();
                        window.location.replace(selected);
                          });
                });

                }
            });
           
            return false;
        }); 
    
    });
</script>

$（文档）.ready（函数（）{
$（'#category'）.change（function（）{
var id=$（this.val（）；
var cat_selected=$（“#类别：selected”）.text（）；
var url='1〕https://www.test.com/course/';
console.log（id）；
$.ajax({
url:“”，
方法：“张贴”，
数据：{id:id}，
async:true，
数据类型：“json”，
成功：功能（数据）{
var html='未选择'；
var i；
对于（i=0；idb->get_where（'t_modules'，array（'t_modules.module_group_id'）=>
$category_id））；
返回$query；
}
控制器：
function get_sub_category($category_id){
    $query = $this->db->get_where('t_modules', array('t_modules.module_group_id' => 
    $category_id));
    return $query;
}

controller: 

    
<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
ini_set('display_errors', 'On');
error_reporting(1);
define('MP_DB_DEBUG', true);
class Site extends CI_Controller {

    /* Index Controller */
    /* ****************************************** */
    public function index()
    {
        $this->view();
    }
    
    
    /* Main Page View */
    /* ****************************************** */
    public function view() {
        $this->load->model('app/courses/course_category_model');
        $this->load->model('display/page_loader');
        
        // get and parse the URL string
        $request_uri = $this->page_loader->getUrlString();
        if($request_uri == '') {
            $request_uri = '/'; 
        }       
        
        //get page content
        
        // To check the logged in status
        $segment = $this->uri->segment(1);
        if($segment == 'login' || $segment == 'password'){
            if ($this->session->userdata('logged_in') === true) {
                redirect(site_url('my-account'));
                exit;
            }
        }
        
        $data = $this->page_loader->loadRequest($request_uri);
        
        if($data['page_status'] == "404"){
            $meta_title = 'Page Not Found';
            $meta_desc = 'Please find what you\'re looking for by using the main menu';
            
            $params = array('page_id' => '0', 'page_url' => '0','meta_title_override' => $meta_title,'meta_desc_override' => $meta_desc);
            $this->load->library("lscms",$params);
            $data["lscms"] = $this->lscms;
            $this->load->view('display/layout-notfound',$data);
        }
        
        else{
            
            $params = array('page_id' => $data["pageId"], 'page_url' => $data["pageUrl"]);
            $this->load->library("lscms",$params);
            $data["lscms"] = $this->lscms;
            
                $data['category'] = $this->course_category_model->get_category()->result();

            
            $this->load->view('display/'.$data["view"], $data);
        }
    }

    public function encrypt_password($start = 0, $end = 0){
        $this->load->library('encrypt');
        if($start && $end){
            $userinfo = $this->db->select('id, userid, passwd')->from('t_users')
                                 ->where("id BETWEEN $start AND $end")
                                 ->where('is_encrypted', '0')
                                 ->order_by('id', 'DESC')
                                 ->get()
                                 ->result();
            // echo '<pre>'; print_r($userinfo); die;
            if(count($userinfo)){
                foreach ($userinfo as $user) {
                    $id = $user->id;
                    $passwd = $user->passwd;
                    $passwd = $this->encrypt->encode($passwd);
                    $data = ['passwd' => $passwd, 'is_encrypted' => '1'];
                    $this->db->where('id', $id)->update('t_users', $data);
                    echo "ID => $id Updated.<br>";
                }
            }
        } else {
            $userinfo = $this->db->select('id, passwd, LENGTH(passwd) as passwd_len')
                                 ->from('t_users')
                                 ->where('userid', 'testcompany123')
                                 ->where('LENGTH(passwd)<', 16)
                                 ->get()
                                 ->row();
            if(count($userinfo)){
                $id     = $userinfo->id;
                $passwd = $userinfo->passwd;
                $passwd = $this->encrypt->encode($passwd);
                $data = ['passwd' => $passwd];
                $this->db->where('id', $id)->update('t_users', $data);
            }
            echo '<pre>'; print_r($userinfo); die;
        }
                                 
    }
    public function get_sub_category(){ 
        $category_id = $this->input->post('id',TRUE);
        $data = $this->course_category_model->get_sub_category($category_id)->result();
        echo json_encode($data);
    }

}

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