Umbraco version: 4.9.1

I want to filter my nodes in the custom data type I created. I use a XPath DropDownlist for this. My problem is that I can't select the parent property id and check this in the XPATH.

Doesn't work:

//Landen/Land [@id= $currentPage/../node[@alias = 'landSelectLand']]/Regio

//Landen/Land [@id= ../data[@alias = 'landSelectLand']]/Regio

Works with static:

//Landen/Land [@id= 1200]/Regio

My scheme:

- Countries
--- Belgium <-- Country (has a country bouned to it (landSelectLand))
---- Ardennes <-- Region
--- The Netherlands <-- Country (has a country bouned to it (landSelectLand))
---- South Holland <-- Region

When i create a document say a country Holland and bound it to the 'Land' The Netherlands, this works, Bellow this country I create a Region, say SH and bound it to South Holland. I want to show only the Regions that are defined below that country.