Challenge 3: Displaying Errors in a Form — Solution

<!-- app/views/products/_form.html.erb -->
<% if product.errors.any? %>
  <div class="errors">
    <ul>
      <% product.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
    </ul>
  </div>
<% end %>

<%= form_with model: product do |f| %>
  <%= f.text_field :name %>
  <%= f.submit %>
<% end %>

Step-by-step trace of an invalid submission:

1. (Chapter 7) The Product model has validates :name, presence: true (or
   similar) declared directly on the model.

2. (Chapter 3) A user submits the form with a blank name. The create
   action runs: @product = Product.new(product_params); if @product.save
   ... else render :new end. Because validation fails, .save returns
   false, so the else branch runs: render :new.

3. (Chapter 7) The failed .save call populated @product.errors
   automatically -- @product itself is still in memory with the user's
   submitted (invalid) values, exactly as it was built from product_params
   moments earlier.

4. (Chapter 3 + this chapter) render :new re-renders the SAME @product
   instance -- no new HTTP request is made, no database re-query happens
   -- straight into app/views/products/new.html.erb, which itself renders
   this chapter's _form.html.erb partial.

5. (This chapter) product.errors.any? is now true, so the error block at
   the top of the partial displays every message from
   product.errors.full_messages, and the form fields below still show the
   user's originally submitted (invalid) values, ready to be corrected and
   resubmitted.

=begin
Notes:
- Nothing in this flow involves a second HTTP request -- render :new
  (as opposed to redirect_to) is specifically what keeps @product's
  in-memory state, including both its errors and the user's typed input,
  intact through to the re-rendered form.
- This end-to-end trace is really the payoff of five separate chapters
  (validations, controllers, views, and this chapter's forms) all working
  together on one single, ordinary "submit an invalid form" interaction.
=end