Quick Start

Let's start with the simplest scenario possible: you have a form with multiple input elements, and when the user clicks on the Submit button you want to display any validation error messages beside the elements that have issues. When the user resolves these issues and clicks the Submit button, the form is processed normal and the page navigates to whatever comes next.

Model

Validations are covered in-depth by the official documentation. Optimism doesn't require anything special.

Optimism is designed for ActiveRecord models that have validations defined, although it should work with any Ruby class that implements Active Model and has an errors accessor.

View

Here's sample form partial for a Post model. It has two attributes - name and body and was generated with a Rails scaffold command.

app/views/posts/_form.html.erb BEFORE Optimism

<%= form_with(model: post, local: true) do |form| %>
  <% if post.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(post.errors.count, "error") %> prohibited this post from being saved:</h2>

      <ul>
        <% post.errors.full_messages.each do |message| %>
          <li><%= message %></li>
        <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= form.label :name %>
    <%= form.text_field :name %>
  </div>

  <div class="field">
    <%= form.label :body %>
    <%= form.text_area :body %>
  </div>

  <div class="actions">
    <%= form.submit %>
  </div>
<% end %>

And here is that same form partial, configured to work with Optimism:

app/views/posts/_form.html.erb AFTER Optimism

<%= form_with(model: post) do |form| %>
  <div class="field">
    <%= form.label :name %>
    <%= form.text_field :name %>
    <%= form.error_for :name %>
  </div>

  <div class="field">
    <%= form.label :body %>
    <%= form.text_area :body %>
    <%= form.error_for :body %>
  </div>

  <div class="actions">
    <%= form.submit %>
  </div>
<% end %>

Eagle-eyed readers will see that setting up a bare-bones Optimism integration requires removing two things and adding one thing to each attribute:

1. Remove local: true from the form_with on the first line

2. Remove the error messages block from lines 2-12 entirely

3. Add an error_for helper for each attribute

The error_for helper creates an empty span tag with an id such as posts_body_error, and this is where the error messages for the body attribute will appear.

Even though form_with is remote-by-default, many developers were confused and frustrated by the lack of opinionated validation handling out of the box for remote forms. Since scaffolds are for new users to get comfortable, remote forms are disabled. This is the primary reason that Optimism was created: we want our tasty remote forms without any heartburn.

Controller

The last step is to slightly modify the create and update actions in our PostsController. The other actions have been removed for brevity:

app/controllers/posts_controller.rb

  def create
    @post = Post.new(post_params)
    respond_to do |format|
      if @post.save
        format.html { redirect_to @post, notice: 'Post was successfully created.' }
        format.json { render :show, status: :created, location: @post }
      else
        format.html { broadcast_errors @post, post_params }
        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
  end

  def update
    respond_to do |format|
      if @post.update(post_params)
        format.html { redirect_to @post, notice: 'Post was successfully updated.' }
        format.json { render :show, status: :ok, location: @post }
      else
        format.html { broadcast_errors @post, post_params }
        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
  end

The only meaningful change required (as seen on lines 8 and 20 in this example) is to replace render :new and render :edit with a call to broadcast_errors which has two mandatory parameters: the model instance and the list of attributes to validate. Usually this is the whitelisted params hash, but you can pass a subset as small as one attribute to be validated.

That's all there is to it. You now have live - if unstyled - form validations being delivered over websockets.