public function order_by( $cols, $order = 'asc' ) 
	{
		if ( !is_array( $cols ) ) 
		{
			$this->orders[] = array( $cols, $order ); return $this;
		}
		else
		{
			foreach( $cols as $key => $col ) 
			{
				if ( is_numeric( $key ) ) 
				{
					$this->orders[] = array( $col, $order );	
				} 
				else 
				{
					$this->orders[] = array( $key, $col );
				}
			}
		}
		
		return $this;
	}