Using Onlyif but dependant on the parent viewmodel?

[jammy-git]

We have a parent/child viewmodel set up so that a parent record can have multiple child records. We need a validation rule on the child record that is only required when a field on the parent has a certain value.

Is this possible?
I've tried this in the child viewmodel:
required: { onlyIf: function () { return parent.FieldX() == Value; } }
Where parent is the self variable passed from the parent viewmodel to the child viewmodel. No errors generated, but this still doesn't work.