gender.rb

class Gender
  include Comparable
  attr_reader :gender
  VALID_ENUMS = [:male, :female]

  def initialize(value)
    value = value.to_s.downcase.to_sym
    @gender = value if VALID_ENUMS.include?(value)
  end

  def ==(other)
    gender == other.gender
  end
  
  def <=>(other)
    VALID_ENUMS.index(gender) <=> VALID_ENUMS.index(other.gender)
  end

  def to_s
    gender.to_s
  end

  def male?
    @gender == :male
  end
  
  def female?
    @gender == :female
  end
  
  def self.male
    new :male
  end

  def self.female
    new :female
  end
end

person.rb

class Person < ActiveRecord::Base
  composed_of :gender
end