自定义模型出错尝试获取非对象Codeigniter 2.x的属性

Question

我尝试修复了一个codeigniter应用程序。在模型中总是说：消息：试图获得非对象的属性

这是我的模特：

class Custom_lesson extends Domain_class
{

    //private $customSftObj = null;
    public $customSftObj = null;
    public $custom_sft = null;
    public function __construct()
    {
        parent::__construct();
    }

    public function getCustomSft()
    {
        if (FALSE == empty($this->customSftObj))
            return $this->customSftObj;

        $customSftDAO = $this->loadDAO('custom_sft');
        $this->customSftObj = $customSftDAO->find($this->custom_sft);
        return $this->customSftObj;
    }

    public function getSft() { 
        return $this->getCustomSft();
    }

    public function doDestroy()
    {
        $custom_sft = $this->getCustomSft();

        if ( $this->hasSM() )
            unlink( custom_sft_path($custom_sft->owner_id, $custom_sft->subject, $custom_sft->grade) . 'lessons/_lesson_' . $this->id . '_SM.pdf' );

        if ( $this->hasTM() )
            unlink( custom_sft_path($custom_sft->owner_id, $custom_sft->subject, $custom_sft->grade) . 'lessons/_lesson_' . $this->id . '_TM.pdf' );
    }

    public function hasSM()
    {
        $custom_sft = $this->getCustomSft();
        return file_exists( custom_sft_path($custom_sft->owner_id, $custom_sft->subject, $custom_sft->grade) . 'lessons/_lesson_' . $this->id . '_SM.pdf');
    }

    public function hasTM()
    {
        $custom_sft = $this->getCustomSft();
        return file_exists( custom_sft_path($custom_sft->owner_id, $custom_sft->subject, $custom_sft->grade) . 'lessons/_lesson_' . $this->id . '_TM.pdf'); 
    }

这是我的自定义dao模型：

<?php 

class Custom_lesson_DAO extends MY_Model
{

    function Custom_lesson_DAO()
    {
        parent::__construct();
    }

    function getTableName()
    {
        return 'custom_table';
    }

    function findForSFT($custom_sft)
    {
        return $this->fetch( array('custom_sft' => $custom_sft->id, 'order_by' => 'listing_order ASC') );
    }

    function findForSFTLimitOffset($custom_sft, $limit, $offset)
    {
        return $this->fetch( array('custom_sft' => $custom_sft->id, 'limit' => $limit, 'offset' => $offset) );
    }

    function countForSFT($custom_sft)
    {
        return $this->count( array('custom_sft' => $custom_sft->id) );
    }

    protected function parseJoin($model)
    {
        if ($model == 'cts_lesson')
        {
            $CI =& get_instance();
            $CI->load->model($model);
            $this->db->join($CI->$model->getTableName(), 'cts_lessons.lesson = lessons.id', 'left');
        }
        else parent::parseJoin($model);

    }

    public function getMaxOrder()
    {
        $this->db->select('MAX(listing_order) as max');
        $max = $this->db->get($this->getTableName())->row()->max;
        return $max;
    }

错误在第42和48行，实际上这些行是相同的函数custom_sft_path

事情是变量没有设置

return file_exists( custom_sft_path($custom_sft->owner_id, $custom_sft->subject, $custom_sft->grade) . 'lessons/_lesson_' . $this->id . '_SM.pdf');

变量总是空的，我不知道：

$custom_sft->owner_id, $custom_sft->subject,

我不知道为什么得到这个错误的帮助我。