Add .error CSS class to all invalid form fields. Rails wraps them in div.fieldWithErrors, which still sucks in 3.0

rails-fields-with-errors-get-class-by-nokogiri.rb

# put this in config/initializer/fields_with_errors.rb

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  if html_tag =~ /<(input|label|textarea|select)/
    error_class = 'error'

    doc = Nokogiri::XML(html_tag)
    doc.children.each do |field|
      unless field['type'] == 'hidden'
        unless field['class'] =~ /\berror\b/
          field['class'] = "#{field['class']} error".strip
        end
      end
    end
    
    doc.to_html.html_safe
  else
    html_tag
  end
end

Hey, I had the old hpricot based patch on which this is based. You've skipped one bit which might be important. It checks to see if the child is an element or not (as in not a text node).

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  if html_tag =~ /<(input|label|textarea|select)/
    doc = Nokogiri::XML(html_tag)
    doc.children.each do |field|
      unless !field.elem? || field['type'] == 'hidden' || field['class'] =~ /\berror\b/
        field['class'] = "#{field['class']} error".strip
      end
    end

    doc.to_html.html_safe
  else
    html_tag
  end
end

The error_class variable isn't required in your version so I've taken that out :)

Thanks for making this a quick and easy upgrade!