_form.html.erb

<%= form_for(@person) do |f| %>
  <% if @person.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@person.errors.count, "error") %> prohibited this person from being saved:</h2>

      <ul>
      <% @person.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :person_name %><br>
    <%= f.text_field :person_name %>
  </div>
  <div id="person_country_id" class="field">
    <%= f.label :country %><br />
    <%= f.collection_select :country_id, Country.order(:name), :id, :name, include_blank: true %>
  </div>
  <div id="person_city_id"class="field">
    <%= f.label :city %><br />
    <%= f.grouped_collection_select :city_id, Country.order(:name), :cities, :name, :id, :city_name, include_blank: true %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>