How to validate if you have a define value in a parent table

Hello,

I have a MainTable with all the general information and a relationship with a ParentTable with all the documents that each project could have.

So I create a form for a task with the MainTable in read-only, and the ParentTable in add/edit capabilities.

This ParentTable, have this attributes: doc_type, doc_file, doc_version.

When a clic on "next" I want to validate in this form that the user have Added a new document with data_type = 'Presentation'. I create a Validation in the form, but it dosen't work.

So what is the correct way to achive this?

Regards