Now that you have seen what Optimism can do, let's flex our muscles a bit and see how far this goes.

app/views/posts/_form.html.erb
<%= form_with(model: post, id: "posts_form") do |form| %>
  <%= form.container_for :name, class: "field" do %>
    <%= form.label :name %>
    <%= form.text_field :name %>
    <%= form.error_for :name, class: "danger hide" %>
  <% end %>
​
  <%= form.container_for :body, class: "field" do %>
    <%= form.label :body %>
    <%= form.text_area :body %>
    <%= form.error_for :body, class: "danger hide" %>
  <% end %>
​
  <div class="actions">
    <%= form.submit %>
  </div>
<% end %>

app/views/posts/_form.html.erb
<%= form_with(model: post, id: "posts_form") do |form| %>
  <%= form.container_for :name, class: "input-group" do %>
    <%= form.label :name %>
    <%= form.text_field :name, class: "form-control" %>
    <%= form.error_for :name, class: "small align-bottom text-danger d-none" %>
  <% end %>
​
  <%= form.container_for :body, class: "input-group" do %>
    <%= form.label :body %>
    <%= form.text_area :body, class: "form-control" %>
    <%= form.error_for :body, class: "small align-bottom text-danger d-none" %>
  <% end %>
​
  <div class="actions">
    <%= form.submit %>
  </div>
<% end %>

Here we introduce the container_for helper, which wraps a form element in a div that has an id similar to the error_for helper, except you're looking at posts_body_container. More importantly, the container receives a CSS class called error that can be used to change the visual characteristics of the error message as well as the input element itself. It is this interplay between cascading style and the order in which the styles are declared that makes this approach work. Consider the following CSS:

.danger {
  color: red;
}
​
.hide {
  display: none;
}
​
.field.error > .hide {
  display: block;
}
​
.field.error > input,
.field.error > textarea {
  background-color: rgba(255, 239, 213, 0.7);
}

.input-group.error > .d-none {
  display: inline !important;
}
​
.input-group.error > input,
.input-group.error > textarea {
  background-color: rgba(255, 239, 213, 0.7);
}

When there is a validation error on an input element, the error message is injected into the span and an error CSS class is added to the container for that element. This results in a cascade where the class responsible for hiding the error message is now flipped to display it instead. In this example, we also show how we can change the visual characteristics of the input elements themselves, in this case making the background color a sickly and distressing translucent peach. It really helps sell the urgency.

If you assign an id to the form itself that matches the expected format - in this case, posts_form - Optimism will also place an invalid class on the form if there are any errors at all. This gives the developer the flexibility to demonstrate an error state at the form level by tweaking how the form is displayed.

Gotchas

Optimism is a fundamentally simple library, which means that inevitably, there's going to be scenarios that require some extra consideration.

Radio buttons and checkboxes

Thanks to a quirk of the way certain OG form input elements work, it's possible for an unchecked element to not be included in a form's parameters. For example, if you have a set of radio buttons with no default selection, your params hash will not include data for it.

Make sure to use the checkbox helpers that Rails provides, which implement a clever hack to make sure that unchecked checkboxes produce a false value in your params.