how to use order by codeigniter

order_by should be after all where commands and should be the last command before retrieving the query ...

thanks for reply,
i changed my order 

PHP Code:

from;
$this->db->select()->from($this->_table_name)->order_by($this->_order_by)->where($array);

to
$this->db->select()->from($this->_table_name)->where($array)->order_by($this->_order_by); 


 but no luck