sort by codeigniter

saya ingin sort data array dengan ketentua city_id yg isinya 1-5 harus tampil duluan dengan penyortiran city_name, dan sisa city_id 6 dan seterusnya juga dengan penyotiran name jugaa.. oh iyaa, city_id dan city_name itu jika di sort ascending beda

- ini model :

function city(){
        $this->db->select('city_name, city_id');
        $this->db->from('city');
        $this->db->where('deleted', '0');
        $this->db->order_by('city_id', 'asc');
        $query = $this->db->get();

        return $query->result_array();
    }

- controller

 public function index(){
        // print_r($this->session->userdata());
        // die();

        $data = array(
            'title' => 'Homepage',
            'page' => 'view_home',
            'js' => array(
                base_url().'assets/js/typehead.js',
                base_url().'assets/js/bootstrap-select.min.js',
                base_url().'assets/js/jquery.vide.js',
                base_url().'assets/plugins/owl/owl.carousel.min.js'
            ),
            'css' => array(
                base_url().'assets/css/bootstrap-select.css',
                base_url().'assets/plugins/owl/assets/owl.carousel.min.css',
                base_url().'assets/plugins/owl/assets/owl.theme.default.min'
            ),
            'categories' => $this->main_model->categories(),
            'city' => $this->main_model->city(),
            'video_banner' => $this->main_model->video_banner(),
            'workspace' => $this->workspace_model->get()
        );

        // print_r($data['video_banner']);die();

        $this->load->view('view_template', $data);
    }

- view

 <select class="selectpicker show-menu-arrow"  name="loc" id="findcity" style="padding: 17px;">
                     <option value="">Pilih Lokasi</option>
                     <?php foreach ($city as $ci){ ?>
                     <option value="<?php echo $ci['city_name']; ?>">
                         <?php echo $ci['city_name']; ?>
                     </option>
                     <?php } ?>
                 </select>