osem-fcy/app/models/difficulty_level.rb

class DifficultyLevel < ActiveRecord::Base
  belongs_to :program
  has_many :events, dependent: :nullify

  has_paper_trail ignore: [:updated_at], meta: { conference_id: :conference_id }

  validates :title, presence: true

  validates :color, format: /\A#[0-9A-F]{6}\z/

  before_validation :capitalize_color

  private

  def capitalize_color
    self.color = color.upcase if color.present?
  end

  def conference_id
    program.conference_id
  end
end