在codeigniter中使用来自helper的模型时，$ db未定义

$CI->load->model(\'stats_model\');
$CI->stats_model->set_hit();

Severity: Notice
Message: Undefined property: Stats_model::$db
Filename: models/stats_model.php
Line Number: 16

$this->db->select(\'*\');

<?php

if (!defined(\'BASEPATH\'))
    exit(\'No direct script access allowed\');

function check_hit() {
//stuff that uses CI
$CI = & get_instance();
$CI->load->library(\'user_agent\');
if ($CI->agent->is_robot()) {
    return FALSE;
} else {

    //check for a 12 hour cookie
    $check = $CI->input->cookie(\'stat\');

    if ($check == false) {
        //insert a database entry
        $CI->load->model(\'Stats_model\');
        $CI->Stats_model->set_hit();

        //set a cookie

        $cookie = array(
            \'name\' => \'stat\',
            \'value\' => \'1\',
            \'expire\' => \'43200\'
        );

       // $CI->input->set_cookie($cookie);
    }
}
}

check_hit();
?>

<?php

if (!defined(\'BASEPATH\'))
exit(\'No direct script access allowed\');

class Stats_model extends Model {

function Stats_model() {
// Call the Model constructor
    parent::Model();
}

function set_hit() {
    $date = date(\'Y-m-d\');

    $this->db->select(\'unique_visitors\');
    $this->db->from(\'daily_stats\');
    $this->db->where(\'date\', $date);
    $query = $this->db->get();
    $date_rows = $query->num_rows();
    $result = $query->row();
    $visits = $result->unique_visitors;
    $visits++;

    $data = array(
        \'unique_visitors\' => $visits,
        \'date\' => $date
    );

    if ($date_rows == 1) {
        $this->db->where(\'date\', $date);
        $this->db->update(\'daily_stats\', $data);
    } else {
        $this->db->insert(\'daily_stats\', $data);
    }
}

}
?>