_form.haml

.col-sm-4
  = f.label :subcategory_id
  = f.select "subcategory_id", @submenu.subcategories.collect { |o| [ o.title, o.id ] }
  %button.btn.btn-secondary{"data-target" => "#createsubcategory", "data-toggle" => "modal", :type => "button"}
    Create Subcategory
            
$('#post_subcategory_id').on('change', function(e){
  console.log("I caught the change of the subcategory")
  var subcatVal = $(this).val();
  var category = $(this).closest("form").find("#post_category_id").val();
  var submenu = $(this).closest("form").find("#post_submenu_id").val();
  $('#new_post').attr('action', '/admin/categories/' + category + "/submenus/" + submenu + '/subcategories/' + subcatVal + '/posts')
})