How do I validates uniqueness of parent of parent?

Hi guys!

I have 3 models: lectures > chapters > lessons

With a has_many through relationship.

The problem: each lecture should have a lesson with the same step for ordering.

In lesson.rb I'm trying to:

validates :step, uniqueness: { scope: :lecture_id }

In the console it seems that lecture_id is nil.

Sorry if the explanation is messy!