function list_categories()
	{
	
		$options = array();
		//select all parent categories first
		$query = $this->db->get_where("category",array('parent' => 0));
		if( $query->num_rows() <= 0 )
			return "<option value='0'>None</option>";
			
		foreach( $query->result() as $row )
		{
			$options[] = "<option value='{$row->id}'>->  {$row->name}</option>";
			
			$subquery = $this->db->get_where("category",array('parent' => $row->id ));
			
			if( $subquery->num_rows() <= 0 )
				return implode("",$options);
			
			foreach( $subquery as $subrow )
			{
				$options[] = "<option value='{$subrow->id}'>---{$subrow->name}</option>";
				$subsubquery = $this->db->get_where("category",array('parent' => $subrow->id ));
				
				if( $subsubquery->num_rows() <= 0 )
					return implode("",$options);
					
				foreach( $subsubquery as $subsubrow )
				{
					$options[] = "<option value='{$subsubrow->id}'>-----{$subsubrow->name}</option>";
				
				}
			}
		}
		return implode("",$options);
	}