user.rb

  EMAIL_PREFERENCES = %w[baby_weight_update_requests in_on_and_around_your_baby nursery marketing_email]
  def wants_email?(email_preference)
    email_preferences.include?(email_preference.to_s)
  end

  def email_preferences=(email_preferences)
    self.email_preferences_mask =
      (email_preferences.map!{|p| p.to_s} & EMAIL_PREFERENCES).map { |p| 2**EMAIL_PREFERENCES.index(p) }.sum
  end

  def email_preferences
    EMAIL_PREFERENCES.reject do |p|
      ((email_preferences_mask || 0) & 2**EMAIL_PREFERENCES.index(p)).zero?
    end
  end