/config/routes.rb
------------------------------------------------------------------------
  get     '/products/update_productsubgroups', as: 'update_productsubgroups'
  get     '/update_productsubgroups' => 'products#update_productsubgroups'
  resources :prodducts

/app/assets/javascript/products.coffee
------------------------------------------------------------------------
$ ->
  $(document).on 'change', '#productgroups_select', (evt) ->
    $.ajax 'update_productsubgroups',
      type: 'GET'
      dataType: 'script'
      data: {
        productgroup_id: $("#productgroups_select option:selected").val()
      }
      error: (jqXHR, textStatus, errorThrown) ->
        console.log("AJAX Error: #{textStatus}")
      success: (data, textStatus, jqXHR) ->
        console.log("Dynamic country select OK!")


/app/views/products/new.html.erb (cascading select)
- dropdown code is the same for views/products/edit.html
------------------------------------------------------------------------

                        <%= f.label :productgroup, "Group", :class => 'col-sm-3 control-label' %>
                        <div class="col-sm-9">
                                <%= f.select :productgroup_id, options_for_select( @productgroups.collect { |productgroup|
                                        [productgroup.name.titleize, productgroup.id] },
                                        @product.productgroup_id), {} ,
                                        { class: 'form-control', id: 'productgroups_select' } %>
                        </div>

                        <%= f.label :productsubgroup, "Sub-group", :class => 'col-sm-3 control-label' %>
                        <div class="col-sm-9">
                                <%= f.select :productsubgroup_id, options_for_select( @productsubgroups.collect { |productsubgroup|
                                        [productsubgroup.name.titleize, productsubgroup.id] },
                                        @productsubgroupschoice), {include_blank: "--Please Select--"} ,
                                        { class: 'form-control', id: 'productsubgroups_select' } %>
                        </div>


/app/views/products/update_productsubgroups.html.erb
------------------------------------------------------------------------
$("#productsubgroups_select").empty()
  .append("<%= escape_javascript(render(:partial => @productsubgroups)) %>")


/app/views/produtsubgroups/_productsubgroups.html.erb
------------------------------------------------------------------------
<option value="<%= productsubgroup.id %>"><%= productsubgroup.name.titleize %></option>