How to implement the output of select( city ) on select( regions) similarly to the grouped_collection_select helper with only individual select s?

Question

Ruby on Rails

<%= f.grouped_collection_select :city_id,
@regions, :cities, :name,
:id, :name
How to display cities based on region.
But the field below(selectom)
Please tell me?

Answer 1

<% @user.ecategories.group_by{|e| e.parent_id}.each do |parent, ecategories| %>
                <%= link_to Ecategory.find(parent).name %>
              <ul class="user_category_ul">
                <% ecategories.each do |ecategory| %>
                  <%= ecategory.name %>,
                  <% end %>
              </ul>
              <% end %>

Something like this