fields_forはこうやって使う - Qiita

belongs_to :item, optional: true #外部キーのnilを許可する mount_uploader :image, ImageUploader def new @item = Item.new @item.images.build end def create @item = Item.new(item_params) redirect_to root_path end def edit @item = Item.find(params[:id]) end def update @item = Item.find(params[:id]) @item.update(update_item_params) redirect_to root_path end def item_params params.require(:item).permit(:name, :in