class Product < ActiveRecord::Base
  validate :must_go_on_sale_before_event_starts

  # has scheduled_for datetime


  protected
  def must_go_on_sale_before_event_starts
    errors.add(:scheduled_for, 'should be before starts at') if scheduled_for >    behaviour.starts_at || behaviour.starts_at < scheduled_for
  end
end

class Product::Event < ActiveRecord::Base
  # has starts_at datetime

Rails validate before save

Rails