osem-fcy/app/models/registration_period.rb

class RegistrationPeriod < ActiveRecord::Base
  belongs_to :conference

  validates :start_date, :end_date, presence: true
  validate :before_end_of_conference
  validate :start_date_before_end_date

  private

  def before_end_of_conference
    errors.
    add(:start_date, "can't be after the conference end date (#{conference.end_date})") if conference && conference.end_date && start_date && (start_date > conference.end_date)

    errors.
    add(:end_date, "can't be after the conference end date (#{conference.end_date})") if conference && conference.end_date && end_date && (end_date > conference.end_date)
  end

  def start_date_before_end_date
    errors.
    add(:start_date, "can't be after the end date") if start_date && end_date && start_date > end_date
  end
end