_list_group.html.erb

<%- closed_or_open = closed_or_open.present? ? closed_or_open : 'closed' -%>
<ul class="events">
  <% events.group_by(&:start_year).each do |year, years_events| %>
  <li><span class="collapsible open"><%= year %></span>
    <ul>
      <% years_events.group_by(&:start_month).each do |month, months_events| %>
      <li><span class="collapsible <%= closed_or_open %>"><%= month %></span>
        <ul>
          <% months_events.each do |event| %>
          <li>
            <%= event_one_liner(event) %>
          </li>
          <% end %>
        </ul>
      </li>
      <% end %>
    </ul>
  </li>
  <% end %>
</ul>