enumerable_validator.rb

require 'forwardable'                                                                                                   
                                                                                                                        
module Hydra                                                                                                            
  module Validations       
    class EnumerableValidator < ActiveModel::EachValidator
      extend Forwardable

      def_delegator :@member_validator, :validate_each, :validate_member
      def_delegator :@member_validator, :kind

      def initialize(member_validator, options = {})
        unless member_validator.is_a? ActiveModel::EachValidator
          raise ArgumentError, "Member validator must be an instance of an ActiveModel::EachValidator subclass"
        end
        @member_validator = member_validator
        super(options)
      end

      def validate_each(record, attribute, value)
        return validate_member(record, attribute, value) unless value.respond_to?(:each)
        if value.respond_to?(:empty?) && value.empty?
          record.errors.add(attribute, "can't be empty") unless options[:allow_empty]
        else
          value.each do |v|
            previous_error_count = record.errors[attribute].size rescue 0
            validate_member(record, attribute, v)
            messages = record.errors[attribute]
            if messages && messages.size > previous_error_count
              record.errors.add(attribute, fixed_message(v, messages.pop))
            end
          end
        end
      end

      protected

      def fixed_message(value, message)
        "value \"#{value}\" #{message}"
      end

    end
  end
end

format_validator.rb

require 'hydra/validations/enumerable_validator'

module Hydra
  module Validations
    class FormatValidator < EnumerableValidator

      def initialize(options = {})
        member_validator = ActiveModel::Validations::FormatValidator.new(options.dup)
	      super(member_validator, options)
      end


      module HelperMethods
        def validates_format_of *attr_names
          validates_with FormatValidator, _merge_attributes(attr_names)
        end
      end

    end 
  end
end