Reference to a Parent value, when creating a Child entry

[MMMiles]

I have what I believe is a syntax issue, but it may be another mistake I have made.

• I have a table Task, which has person_id , which is a Ref to table Person[id] (required "primary contact")
• The table Contact, is a part of Task, and each row has a person_id, which is Ref to Person[id] (optional "secondary contacts")
• Everything is working as intended, except when I want to validate the person_id of a child Contact

Task and Contact are created in the same form. person_id for the parent Task is always chosen first, and then child Contact entries can be added.

In Contact, my intention was to make the person_id VALID_IF =  "All Person, except the person_id of the parent Task"

[task_id].[person_id] appears to yield the correct value from the parent Task, but I don't understand how to use it in the valid_id formula. Or I am going about this in the wrong way.

Any help is appreciated