Hello members of the friendliest community on the web!

I've been learning Umbraco for about 3 months. Currently, I'm something stuck with achieving dependency between two properties in the same document type. I want a MNTP just loading the children of a previously selected node (parent) in another MNTP. I'm currently using Umbraco version 8.6.1. I have followed exactly what is indicated in this article, but I haven't been able to make it work.

In Content section, you can suppose my structure is:

DataModel (alias: modelFolders)
    - DataSchool (alias: schoolFolders)
    - - Schools (alias: schoolFolder)
    - - - A (doctype alias: school, node id: 1000)
    - - - - A1 (doctype alias: subSchool)
    - - - - A2 (doctype alias: subSchool)
    - - - B (doctype alias: school, id: node 1001)
    - - - - B1 (doctype alias: subSchool)
    - - - - B2 (doctype alias: subSchool)

In the first MNTP (with alias: pickerSchool) my xpath expression would be something like:

$root/modelFolders/schoolFolders/schoolFolder

Allow items of type = school

Which allows me to select A or B.

According to the aforementioned blogpost, in the second MNTP (with alias: pickerSubSchool) my xpath expression would be something like:

$root/modelFolders/schoolFolders/schoolFolder/school[@id=./ancestor-or-self::node()[@id=./@id]/pickerSchool/text()]

For example, if A is selected in the first MNTP, then the second MNTP should show me A1 and A2 (to choose between them), but it doesn't happen like that and it shows the entire content tree.

If I hardcoded the id of one of the nodes, for example:

$root/modelFolders/schoolFolders/schoolFolder/school[@id=1000]

then, it correctly shows A1 and A2 to select.

Obviously, the desired solution should be dynamic (not hardcoded).

Thank you in advance for any help or guidance you can give me on this issue.

Regards, Dovier