example.rb

# -*- encoding : utf-8 -*-
module BookingRequestEvent::HasOriginator
  extend ActiveSupport::Concern

  # you could add :crew if you needed it later
  VALID_ORIGINATORS = {
    :person => "Person",
    :operator => "User"
  }

  module ClassMethods
    def validate_originators(*list)
      @originators = list

      validate :has_one_originator

      associate_originators
    end


    def originators
      @originators ||= []
    end

    private

    def associate_originators
      @originators.each do |originator|
        raise "Originator not valid" unless VALID_ORIGINATORS.include?(originator)
        belongs_to originator, :class_name => VALID_ORIGINATORS[originator]
      end
    end

  end

  def originators
    self.class.originators
  end

  def originator
    @originator ||= originators.detect { |originator| self.send(originator) != nil }
  end

  def originator_is_person?
    self.originator == :person
  end

  def originator_is_operator?
    self.originator == :operator
  end
  
  def has_one_originator

    return if originators.one?() { |originator| self.send(originator) != nil }

    if originators.length == 1
      errors.add(originators.first, "is the required originator")
    else
      errors.add(:originators, "must have one, and only one, specified")
    end
  end

end