Add an error class to invalid fields and labels, overriding the standard Rails behaviour of wrapping them in div.field_with_errors

field_with_error.rb

# /config/initializers/field_with_error.rb
# Based on https://gist.github.com/niklas/772018

ActionView::Base.field_error_proc = proc do |html_tag, _instance|
  if html_tag.match? %r{<(input|label|textarea|select)}
    error_class = "has-error"

    doc = Nokogiri::XML(html_tag)
    doc.children.each do |field|
      next if field["type"] == "hidden"
      next if field["class"].match? %r{\b#{error_class}\b}
      field["class"] = "#{field['class']} #{error_class}".strip
    end

    doc.to_html.html_safe
  else
    html_tag
  end
end