application_helper.rb

def field_with_errors(object, method, &block)
    if block_given?
      if object.errors[method].empty?
        content = with_output_buffer(&block)
        content.html_safe
      else
        content = with_output_buffer(&block)
        content_tag(:span, content, :class => "field_with_errors")
      end
    end
  end

  def textarea_with_errors(object, method, &block)
    if block_given?
      if object.errors[method].empty?
        content = with_output_buffer(&block)
        content.html_safe
      else
        content = with_output_buffer(&block)
        content = content.gsub(/class=\"#{method}_box\"/, "class=\"#{method}_box textarea_with_errors\"")
        content.html_safe
      end
    end
  end