ajax_helper_ornek_kullanım.html.erb

<div class='form-group col-sm-6'>
  <%= select_tag(:country_id, options_from_collection_for_select(Country.all, :id, :name), class: 'form-control') %>
</div>
<div class='form-group col-sm-6'>
  <%= select_tag(:city_id, [], disabled: true, class: 'form-control') %>
</div>
<div class='form-group col-sm-6'>
  <%= select_tag(:district_id, [], disabled: true, class: 'form-control') %>
</div>

<script type="text/javascript">
  $(document).ready(function() {
    $('#country_id, #city_id, #district_id').select2();

    var parameters = [
      {
        el: '#country_id', // Zorunlu
        target: '#city_id',  // Zorunlu
        params: { 'country_id': '#country_id' },  // source içerisinde parametre kullanılacaksa zorunlu
        source: '/api/locations/countries/:country_id/cities/', // Zorunlu
        value_attribute: 'id', // Opsiyonel, default: id
        label_attribute: 'name', // Opsiyonel, default: name
        reset_selectors: '#city_id, #district_id', // Opsiyonel
        placeholder: 'Şehir Seçiniz' // Opsiyonel
      },
      {
        el: '#city_id',
        target: '#district_id',
        params: { 'city_id': '#city_id' },
        source: '/api/locations/cities/:city_id/districts',
        value_attribute: 'name',
        reset_selectors: '#district_id',
        placeholder: 'İlçe Seçiniz'
      },
    ]
    var ajax_helper_for_selectbox = new ajaxHelper(parameters)
    ajax_helper_for_selectbox.init()
  });
</script>