ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  if html_tag.match('class')
    html_tag.gsub(/class="(.*?)"/, 'class="\1 field_with_errors"').html_safe
  else
    parts = html_tag.split('>', 2)
    parts[0] += ' class="field_with_errors">'
    (parts[0] + parts[1]).html_safe
  end
end