ar_messages.rb

module ActiveRecord
  class Errors

    # Redefine the ActiveRecord::Errors::full_messages method:
    #  E.g. validates_acceptance_of :accepted_terms, :message => 'Please accept the terms of service'
    def full_messages
      full_messages = []

      @errors.each_key do |attr|
        @errors[attr].each do |msg|
          full_messages << msg unless msg.nil?
        end
      end

      return full_messages
    end
  end
end

module ApplicationHelper
  def validation_errors(object, summary = nil)
    return if object.nil? || object.errors.empty?

    content_tag(:div) do
      html = content_tag(:h4, summary || 'The form was filled with the errors', :class => 'errors')
      html << content_tag(:ul) do
        object.errors.full_messages.map { |msg| content_tag(:li, msg) }.join()
      end
    end
  end
end