id_enumeration.rb

module IdEnumeration
  def acts_as_enumeration(*ordered_list_of_values)
    ordered_list_of_values.map(&:to_s).each_with_index do |value, index|
      const_set(value.upcase, index + 1)

      self.class.send(:define_method, "find_#{value}!") do
        find(const_get(value.upcase))
      end

      self.class.send(:define_method, "find_#{value}") do
        find_by_id(const_get(value.upcase))
      end

      define_method "#{value}?" do
        id == self.class.const_get(value.upcase)
      end
    end
  end
end

ActiveRecord::Base.send(:extend, IdEnumeration)

product_state.rb

class ProductState < ActiveRecord::Base
  acts_as_enumeration :new, :active, :locked, :sold, :expired, :removed, :deleted
end