我有一個創建產品的 Rails 簡單表單，表單中有以下三個字段，以便將產品與適當的類別相關聯： <div class="form-group">   <%= f.input :child_category_id, :collection => @categories.order(:name), :label_method => :name, :value_method => :id, label: "Category", :include_blank => "--Select Category--", input_html: { id: "first_dropdown" } %> </div> <div class="show_hide">   <%= f.input :subcategory_id, :collection => [] || @subcategory.order(:name), :label_method => :name, :value_method => :id, label: false, :include_blank => "--Select Category--", input_html: { id: "second_dropdown" } %> </div> <div class="show_hide_third">   <%= f.input :child_subcategory_id, :collection => [] || @child_subcategory.order(:name), :label_method => :name, :value_method => :id, label: false, input_html: { id: "third_dropdown" } %> </div>這是路線：resources :products, :path => "products/select_item", only: [:select_item] do  get :select_item, on: :collectionend這是我擁有的控制器內部的方法：def select_item  if params[:child_category_id].present?    @subcategory = Subcategory.where(child_category_id: params[:child_category_id])    render :json => @subcategory.order(:name)  end  if params[:subcategory_id].present?    @child_subcategory = ChildSubcategory.where(subcategory_id: params[:subcategory_id])    render :json => @child_subcategory.order(:name)    endend這是控制器內部的新產品和創建產品方法：def new; enddef create  @product = Product.new(product_params)    respond_to do |format|      if @product.save        format.html { redirect_to root_path }        format.json { render :new, status: :created, location: @product }        flash[:success] = "Product was successfully added."      else        format.html { render :new }        format.json { render json: @product.errors, status: :unprocessable_entity }      end    endend 一切正常，所有字段都正確填充。我遇到的唯一問題是，當表單再次呈現時，由于驗證錯誤，JSON 字段顯示為空，即使它們已正確填充。即使表單返回驗證錯誤，我如何保持填充 JSON 表單字段？