Here’s how you can get character count, error validation, and makdown preview while you type, without page refresh, without extra JS

live error validation and markdown previews

The trick: #

  1. let the form have a second submit button with a different URL
  2. submit the different URL
  3. the different URL will respond with a turbo_stream


rails generate scaffold Message content:text
rails g stimulus form
# app/models/message.rb
  validates :content, presence: true
  validates :content, length: { in: 5..1000 }
  • div id message_preview
  • hidden button with formatction to a different url
  • submit the hidden button oninput (with a stimulus controller)
# app/views/messages/_form.html.erb
<%= form_with(model: message, data: { controller: "form", action: "input->form#remotesubmit" }) do |form| %>
    <%= form.label :content, style: "display: block" %>
    <%= form.text_area :content %>

  <div id="message_preview">
    <%= markdown message.content %>

    <%= form.button "Preview Message", formaction: preview_messages_path, name: "_method", value: "post", data: { form_target: "submitbtn" } %>
    <%= form.submit %>
<% end %>

The Stimulus controller to:

  • hide the second submit button
  • autosubmit whenever there are any changes
// app/javascript/controllers/form_controller.js
import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  static targets = ["submitbtn"]

  // hide the submit button
  connect() {
    this.submitbtnTarget.hidden = true

  // click the hidden button -> submit the form
  remotesubmit() {

  // same as above, but with "debounce"
  // remotesubmit() {
  //   clearTimeout(this.timeout)
  //   this.timeout = setTimeout(() => {
  //     }, 500)
  // }

So now we try to send a request to the server each time we change something in the form…

We need a way to respond to it!

  • create a new route that will respond to the second form submit button
# config/routes.rb
  resources :messages do
    collection do
      post :preview
  • create another message object from the submitted params
  • respond with a turbo_stream
# app/controllers/messages_controller.rb
  def preview
    # params.dig(:message, :content) 
    @preview =

    respond_to do |format|
  • update the message_preview with the attributes from the @preview object
  • render errors, sanitized @preview.content, or anything based on the @preview object
# app/views/messages/preview.turbo_stream.erb
<%= turbo_stream.update "message_preview" do %>
  <%= @preview.content.length %>
  <%= @preview.valid? %>
  <%= @preview.errors.full_messages %>
  <%= @preview.attributes %>
  <%= simple_format @preview.content %>
<% end %>

The drawback of this approach is having to exchange MANY request-responces with the server. Unlike a pure JS approach, that would handle everything on the client side.

This is inspired by the amazing idea of a form having a second submit button to a different URL, that was described in Thoughtbot’s: Server-rendered live previews