Parse error: syntax error, unexpected T_OBJECT_OPERATOR

[eluser]Pensacola1989[/eluser]
I'm new to codeigniter ,get some problems,asking for help ,Thanks!

Code:

<?php
    class Mhome extends Model
    {
       function Mhome()
       {
         parent::Model();
       }
       function get_category()
       {
         $query=$this->db->query('SELECT * FROM category');
         return $query->result();
       }
       function get_article($category_id)
       {
         $query=$this-db->query("SELECT * FROM article WHERE category_id=$category_id");
         return $query->result();
       }
    }

the error message:
Parse error: syntax error, unexpected T_OBJECT_OPERATOR in D:\wamp\www\demo\system\application\models\Mhome.php on line 15