Massimo,
I think you need one more association – from Task to Project, like this:
Then your page will look like this
And in the Subproject reference selector, you can use the Constrained By feature
Note that the association between task and project is needed to constrain the subprojects.
Hope that helps,
Mike
Thank you Mike.
I understand this is probably the only way to achieve it in Mendix, and keep using the available tools for a quick interface deployment. Definitely not a perfect solution from a database standpoint.