我有一个国家的选择字段，一个给定国家的航运选项第二个选择字段，当第一个国家和第二次航运已被选中，我想显示航运成本。
当我第一次用show加载shippingservices时，shippingservices资源被正确地Map到第一个国家（只有特定权重和所选国家的服务），但是当我对ajax资源的update\u shipping操作使用相同的查询时，我得到 postzones.land_id = NULL 我的控制器：

class CartsController < ApplicationController

  def show
    @cart = Cart.find(params[:id])
    @shippingservices = @cart.available_shipping_services.includes(:lands, :postzones).where('postzones.land_id = ?', Land.first.id)
  end

  def update_shipping
    @cart = Cart.find(params[:id])
    @shippingservices = @cart.available_shipping_services.includes(:lands, :postzones).where('postzones.land_id = ?', params[:land_id])
    respond_to do |format|
      format.js
    end
  end
end

在我的手推车模型中，我有：

def available_shipping_services
  Shippingservice.where('weightmin <= ? and weightmax >= ?', total_weight, total_weight).where('shippingservices.shippingcarrier = ?', '1') 
end

javascript ajax部分得益于@rewrited:

$(document).on("change", "#lands_select", function(event){
  $.ajax({
    url: "/carts/update_shipping/" + event.target.value,
    type: "GET"
  })
});

app/views/carts/update_shipping.js.erb是：

$("#shippingservices_select").empty()
  .append("<%= escape_javascript(render(:partial => @shippingservices)) %>")

app/views/carts/\u shippingservices.html.erb是：

<option value="<%= shippingservice.id %>"><%= shippingservice.name.titleize %></option>

当我尝试调用ajax时，我得到：

Started GET "/carts/update_shipping/2" for 127.0.0.1 at Tue Dec 18 14:17:21 +0100 2018
Processing by CartsController#update_shipping as */*
  Parameters: {"id"=>"2"}
  Cart Load (0.4ms)  SELECT `carts`.* FROM `carts` WHERE `carts`.`id` = ? LIMIT 1  [["id", 5]]
  Cart Load (0.2ms)  SELECT `carts`.* FROM `carts` WHERE `carts`.`id` = ? LIMIT 1  [["id", "2"]]
  CartItem Load (0.4ms)  SELECT `cart_items`.* FROM `cart_items` WHERE `cart_items`.`cart_id` = 2
  SQL (0.9ms)  SELECT `shippingservices`.`id` AS t0_r0, `shippingservices`.`serviceID` AS t0_r1, `shippingservices`.`title` AS t0_r2, `shippingservices`.`name` AS t0_r3, `shippingservices`.`shippingcarrier` AS t0_r4, `shippingservices`.`length` AS t0_r5, `shippingservices`.`width` AS t0_r6, `shippingservices`.`height` AS t0_r7, `shippingservices`.`weightmin` AS t0_r8, `shippingservices`.`weightmax` AS t0_r9, `shippingservices`.`price` AS t0_r10, `shippingservices`.`created_at` AS t0_r11, `shippingservices`.`updated_at` AS t0_r12, `lands`.`id` AS t1_r0, `lands`.`name` AS t1_r1, `lands`.`partof` AS t1_r2, `lands`.`continent` AS t1_r3, `lands`.`area` AS t1_r4, `lands`.`CallingCode` AS t1_r5, `lands`.`ISOcode2` AS t1_r6, `lands`.`ISOcode3` AS t1_r7, `lands`.`created_at` AS t1_r8, `lands`.`updated_at` AS t1_r9, `postzones_shippingservices`.`id` AS t2_r0, `postzones_shippingservices`.`land_id` AS t2_r1, `postzones_shippingservices`.`shippingservicezone` AS t2_r2, `postzones_shippingservices`.`shippingservice_id` AS t2_r3, `postzones_shippingservices`.`created_at` AS t2_r4, `postzones_shippingservices`.`updated_at` AS t2_r5 FROM `shippingservices` LEFT OUTER JOIN `postzones` ON `postzones`.`shippingservice_id` = `shippingservices`.`id` LEFT OUTER JOIN `lands` ON `lands`.`id` = `postzones`.`land_id` LEFT OUTER JOIN `postzones` `postzones_shippingservices` ON `postzones_shippingservices`.`shippingservice_id` = `shippingservices`.`id` WHERE (weightmin <= 0 and weightmax >= 0) AND (shippingservices.shippingcarrier = 'Poste italiane') AND (postzones.land_id = NULL)
  Rendered collection (0.0ms)
  Rendered carts/update_shipping.js (3.2ms)
Completed 200 OK in 13ms (Views: 7.6ms | ActiveRecord: 1.8ms)

我想所有的设置都是正确的，但是控制器无法解析活动记录查询，因为他正在查找 (postzones.land_id = NULL) .
如何获得如下内容：postzones.land\u id=parameters:{“id”=>“2”}。这意味着所有的运输服务，其中尊重重量的车的国家与id:2？
提前谢谢大家。

更新：

在我的app/views/carts/show.html.erb中，我有以下表单：

<%= form_for :cart, :url => {:action => "update_shipping"}, :html => { :method => "get", :remote => true } do |f| %>
  <%= f.select(:land_id, options_for_select(@lands.collect { |l| [l.name, l.id]}, 0), {}, {:prompt => "select a country", :id => 'lands_select'}) %><br>
  <%= f.select(:shippingservice_id, options_for_select(@shippingservices.collect { |s| [s.title.titleize, s.id] }, 0), {}, {:id => 'shippingservices_select', :prompt => "select a carrier"}) %><br>
  <%= f.submit "Calculate shipping" %><br>
<% end %>

carts\u控制器的“显示”页面显示当前装运车。carts模型通过carts\u项来知道它的重量。这允许我在模型中计算购物车的总重量，并在模型中进行查询，从而生成可用的装运服务，我在显示页面中使用该服务成功地创建了第一个加载，其中第一个条目显示了所有可用的解决方案，但之后的每一个更改都会导致第二个选择器窗体无效。
这个 {:action => "update_shipping"} 可能是错误的，以后必须更改为另一个操作，该操作应显示生成的运输成本，但目前我正在集中精力使两个select字段正常工作。
又是唐卡！