How to get fields provided by a behavior from an invariant?

Hi plone devs,

I'm struggling with a problem that I reported on stackoverflow but was
pointed to ask here:
http://stackoverflow.com/questions/23021989

The TL;DR; version is:

I have a behavior with two fields week and year, on a content type (that
has that behavior) I create an invariant because I want those two fields to
be mandatory (but only for that specific content type).

I thought that an invariant on the content type checking for the values of
the week and year field would be the simplest solution but seems that for
some reason the invariant is only aware of the values of the interface is
defined within.

Maybe because the invariant comes from z3c.form and behaviors and so on
from dexterity?

What solution would you suggest? I need to validate the week and year
fields for that specific content type.

Cheers,
Gil

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