PHP的CI框架插入数据问题

今天用了CI框架对数据库进行insert一直不成功,不知道问题出在哪里了.

 public function add()
    {
        $cn_name = $_POST['cn_name'];
        $en_name = $_POST['en_name'];
        $nt_code = $_POST['nt_code'];
        $cr_code = $_POST['cr_code'];
        $level = $_POST['level'];
        $data=array();
        $data['cr_code'] = $cr_code;
        $data['cn_name'] = $cn_name;
        $data['en_name'] = $en_name;
        $data['nt_code'] = $nt_code;
        $data['level'] = $level;
        $res = $this->dict_model->addDict($data);
        $redData = array(
            "res" => $res,
            'msg' => '添加成功'
        );
        echo json_encode($redData);
    }

model层:

 public function addDict($data)
       {
        $res = $this->db->insert('tbl_hscode_dict',$data);
           return $res;
        }

一直返回false

调用$this->db->getLastSql()打印出sql语句进行排查