Rails 4，尝试访问jbuilder模板中的对象属性时没有方法错误答案

【问题标题】：Rails 4, No Method Error when attempting to access an objects attributes in jbuilder templateRails 4，尝试访问jbuilder模板中的对象属性时没有方法错误
【发布时间】：2016-07-16 18:02:05
【问题描述】：

我正在使用 jbuilder 构建一个 json 响应。见下文，我的 jbuilder 模板：

json.array! @items do |item|
    json.title                          item.title
    json.description                    item.description
    json.category                       item.categories.first.title
    json.price                          item.price / 100.0
    json.status                         item.status

    if !item.banned?
        json.published_date             item.published_date
        json.seller_name                item.seller.name
    end

    json.seller_latitude        item.seller.latitude.to_f
    json.seller_longtitude      item.seller.longtitude.to_f
end

我可以像这样从 item.categories 集合中访问一个对象，没有任何问题：

item.categories.first

当我尝试从上面返回的类别对象中提取标题属性时，我在浏览器中收到此错误：

undefined method 'title' for nil:NilClass

我在我的 jbuilder 模板中的这一行之前放了一个 byebug 语句：

item.categories.first.title

在 byebug 控制台中，我可以访问 title 属性没问题。请参阅下面的 byebug 控制台打印输出：

Started GET "/items.json" for ::1 at 2016-07-16 13:59:45 -0400
Processing by ItemsController#index as JSON
  Item Load (0.5ms)  SELECT "items".* FROM "items"

[1, 10] in /Users/Macbook/projects/VSC/app/views/items/index.json.jbuilder
    1: 
    2: json.array! @items do |item|
    3:  json.title                          item.title
    4:  json.description                item.description
    5:  byebug
=>  6:  json.category                       item.categories.first.title
    7:  json.price                          item.price / 100.0
    8:  json.status                         item.status
    9: 
   10:  if !item.banned?
(byebug) item.categories.first.title
  Category Load (0.3ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 1]]
"movies"
(byebug)

我被难住了，我做错了什么？

编辑：

这里是jbuilder渲染视图时触发的sql：

Started GET "/items.json" for ::1 at 2016-07-16 14:24:29 -0400
Processing by ItemsController#index as JSON
  Item Load (0.8ms)  SELECT "items".* FROM "items"
  Category Load (0.5ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 1]]
  User Load (0.4ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.5ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 2]]
  CACHE (0.1ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.4ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 3]]
  CACHE (0.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.5ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 4]]
  CACHE (3.8ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.5ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 5]]
  CACHE (0.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.4ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 6]]
  CACHE (0.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.5ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 7]]
  CACHE (0.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.4ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 8]]
  CACHE (0.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.5ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 9]]
  CACHE (0.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.4ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 10]]
  CACHE (0.2ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1  [["id", 1]]
  Category Load (0.4ms)  SELECT  "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1  ORDER BY "categories"."id" ASC LIMIT 1  [["item_id", 11]]
  Rendered items/index.json.jbuilder (317.5ms)
Completed 500 Internal Server Error in 329ms (ActiveRecord: 11.6ms)

ActionView::Template::Error (undefined method `title' for nil:NilClass):
    2: json.array! @items do |item|
    3:  json.title                          item.title
    4:  json.description                item.description
    5:  json.category                       item.categories.first.title
    6:  json.price                          item.price / 100.0
    7:  json.status                         item.status
    8: 
  app/views/items/index.json.jbuilder:5:in `block in _app_views_items_index_json_jbuilder___331360250519835275_70269069934300'
  app/views/items/index.json.jbuilder:2:in `_app_views_items_index_json_jbuilder___331360250519835275_70269069934300'

【问题讨论】：

好的，你能检查一下Jbuilder渲染视图时它触发的SQL吗？该 SQL 将是一个很好的调试起点
我贴出jbuilder渲染视图时触发的SQL，请看
好的，好的。我现在可以看到 ID 为 1 到 11 的项目有 categories。 id 11 之后的下一项是破坏代码。现在，打开rails console 和具有id 的项目可能为12，我确信它没有与之关联的类别。这就是问题所在。因此，您需要进行一些过滤以仅选择具有类别的项目。
解决了！谢谢@ArupRakshit

标签： ruby-on-rails json jbuilder

【解决方案1】：

您看到item.categories.first 存在，这确实意味着another_item.categories.first 始终存在，而another_item 是循环中的item：

json.array! @items do |item|
  json.category item.categories.first.title
end

请在控制台中检查以查找哪些项目没有任何类别： Item.all.find_all{ |item| item.categories.empty? }

【讨论】：