# Place this code in a initializer. E.g: config/initializers/form_error.rb
ActionView::Base.field_error_proc = Proc.new do |html_tag, instance_tag|
  fragment = Nokogiri::HTML.fragment(html_tag)
  field = fragment.at('input,select,textarea')

  model = instance_tag.object
  error_message = model.errors.full_messages.join(', ')

  html = if field
           field['class'] = "#{field['class']} invalid"
           html = <<-HTML
              #{fragment.to_s}
              <p class="error">#{error_message}</p>
           HTML
           html
         else
           html_tag
         end

  html.html_safe
end

# config/initializers/fields_with_errors.rb
ActionView::Base.field_error_proc = proc do |html_tag, instance|
    html_doc = Nokogiri::HTML::DocumentFragment.parse(html_tag, Encoding::UTF_8.to_s)
    element = html_doc.children[0]

    if element
        element.add_class('is-invalid')

        if %w[input select textarea].include? element.name
            instance.raw %(#{html_doc.to_html} <div class="invalid-feedback">#{[*instance.error_message].to_sentence}</div>)
        else
            instance.raw html_doc.to_html
        end
    else
        html_tag
    end
end