Rendering events in a form

Since events are ActiveModel objects, form_with and the standard error helpers work exactly as they do with ActiveRecord. Pass the event as the model and Rails takes care of field population and error display:

<%# app/views/debts/new.html.erb %>
<%= form_with model: @event, url: debt_events_path do |f| %>
  <% if @event.errors.any? %>
    <ul>
      <% @event.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
    </ul>
  <% end %>

  <div>
    <%= f.label :amount %>
    <%= f.number_field :amount %>
  </div>
  <div>
    <%= f.label :interest_rate, "Interest rate" %>
    <%= f.number_field :interest_rate, step: "0.01" %>
  </div>
  <div>
    <%= f.label :at, "Issued at" %>
    <%= f.datetime_field :at %>
  </div>

  <%= f.submit "Issue debt" %>
<% end %>

To show a message inline next to a specific field, scope the lookup by attribute with the standard errors[:attr] API — the same idiom you’d use on any ActiveRecord model:

<div>
  <%= f.label :amount %>
  <%= f.number_field :amount %>
  <% if @event.errors[:amount].any? %>
    <span class="error"><%= @event.errors[:amount].full_messages %></span>
  <% end %>
</div>

Funes events actually expose three error collections — errors, own_errors, and state_errors — to keep different failure modes apart. The errors used above is the right pick most of the time and the standard Rails idiom. See Reading the right error collection for when to reach for each.