ActiveRecord .last 和 ActiveRecord .last(1) 返回不同的结果答案

【问题标题】：ActiveRecord .last and ActiveRecord .last(1) return different resultsActiveRecord .last 和 ActiveRecord .last(1) 返回不同的结果
【发布时间】：2015-02-23 16:37:12
【问题描述】：

在 MySQL 5.5 中使用 ActiveRecord 4.2

我有一个名为 Product 的模型。

在控制台中运行Product.last - 我得到了表中的第一条记录（id 为 1，2013 年为 updated_at）

正在运行 Product.last(1) - 我在 2015 年 2 月获得了表中 ID 为 (122039) 和 updated_at 的 LAST 记录。

我的印象是它们应该返回相同的值。为什么Product.last 会返回Product.first 的结果？

附：这是一个开发数据库。所以我销毁它并重新迁移它并重新播种所有数据。不知道这是否重要，但为了加快播种过程（因为我用 100,000 行生产级数据播种了一些表） - 我使用批量插入和 ActiveRecord::Base.connection.execute(sql_from_file)

从 .sql 文件中播种

更新

进行了比较（请参阅下面的 2 个编辑），Product.last 似乎没有做order by id desc，而 Product.last(1) 确实做到了。任何想法为什么？

编辑 1 来自日志：

Product.last (1)

    SELECT DISTINCT 
  `products`.`id` 
FROM
  `products` 
  LEFT OUTER JOIN `restrictions` 
    ON `restrictions`.`id` = `products`.`distribution_restriction_id` 
    AND `restrictions`.`type` IN ('DistributionRestriction') 
  LEFT OUTER JOIN `vendors` 
    ON `vendors`.`id` = `products`.`vendor_id` 
  LEFT OUTER JOIN `manufacturers` 
    ON `manufacturers`.`id` = `products`.`manufacturer_id` 
  LEFT OUTER JOIN `categories` 
    ON `categories`.`id` = `products`.`category_id` 
    AND `categories`.`type` IN ('ProductCategory') 
  LEFT OUTER JOIN `products_restrictions` 
    ON `products_restrictions`.`product_id` = `products`.`id` 
  LEFT OUTER JOIN `restrictions` `restrictions_products` 
    ON `restrictions_products`.`id` = `products_restrictions`.`restriction_id` 
    AND `restrictions_products`.`type` IN ('ProductRestriction') 
  LEFT OUTER JOIN `categories` `departments_products` 
    ON `departments_products`.`id` = `products`.`department_id` 
    AND `departments_products`.`type` IN ('Department') 
  LEFT OUTER JOIN `comments` 
    ON `comments`.`commentable_id` = `products`.`id` 
    AND `comments`.`commentable_type` = 'Product' 
  LEFT OUTER JOIN `pictures` 
    ON `pictures`.`picturable_id` = `products`.`id` 
    AND `pictures`.`picturable_type` = 'Product' 
ORDER BY `products`.`id` DESC 
LIMIT 1 ;

SELECT 
  `products`.`id` AS t0_r0,
  `products`.`name` AS t0_r1,
  `products`.`client_number` AS t0_r2,
  `products`.`client_reference_string` AS t0_r3,
  `products`.`client_id` AS t0_r4,
  `products`.`vendor_number` AS t0_r5,
  `products`.`vendor_reference_string` AS t0_r6,
  `products`.`vendor_id` AS t0_r7,
  `products`.`manufacturer_number` AS t0_r8,
  `products`.`manufacturer_reference_string` AS t0_r9,
  `products`.`manufacturer_id` AS t0_r10,
  `products`.`reman_number` AS t0_r11,
  `products`.`superseded_by_id` AS t0_r12,
  `products`.`description` AS t0_r13,
  `products`.`category_id` AS t0_r14,
  `products`.`first_year_used` AS t0_r15,
  `products`.`last_year_used` AS t0_r16,
  `products`.`reorder_level` AS t0_r17,
  `products`.`minimum_order_quantity` AS t0_r18,
  `products`.`total_stock` AS t0_r19,
  `products`.`stock_details` AS t0_r20,
  `products`.`status` AS t0_r21,
  `products`.`discontinued_at` AS t0_r22,
  `products`.`currency` AS t0_r23,
  `products`.`dealer_exchange_amount_cents` AS t0_r24,
  `products`.`dealer_sale_amount_cents` AS t0_r25,
  `products`.`retail_exchange_amount_cents` AS t0_r26,
  `products`.`retail_sale_amount_cents` AS t0_r27,
  `products`.`cost_amount_cents` AS t0_r28,
  `products`.`core_charge_amount_cents` AS t0_r29,
  `products`.`reman_amount_cents` AS t0_r30,
  `products`.`out_of_warranty` AS t0_r31,
  `products`.`backordered_till` AS t0_r32,
  `products`.`repair_only` AS t0_r33,
  `products`.`publicly_visible` AS t0_r34,
  `products`.`period_usage` AS t0_r35,
  `products`.`distribution_restriction_id` AS t0_r36,
  `products`.`department_id` AS t0_r37,
  `products`.`stats` AS t0_r38,
  `products`.`announcement_date` AS t0_r39,
  `products`.`status_with_vendor` AS t0_r40,
  `products`.`masterfile_details` AS t0_r41,
  `products`.`created_at` AS t0_r42,
  `products`.`updated_at` AS t0_r43,
  `restrictions`.`id` AS t1_r0,
  `restrictions`.`name` AS t1_r1,
  `restrictions`.`code` AS t1_r2,
  `restrictions`.`description` AS t1_r3,
  `restrictions`.`severity` AS t1_r4,
  `restrictions`.`begins_at` AS t1_r5,
  `restrictions`.`expires_at` AS t1_r6,
  `restrictions`.`type` AS t1_r7,
  `restrictions`.`created_at` AS t1_r8,
  `restrictions`.`updated_at` AS t1_r9,
  `vendors`.`id` AS t2_r0,
  `vendors`.`name` AS t2_r1,
  `vendors`.`code` AS t2_r2,
  `vendors`.`created_at` AS t2_r3,
  `vendors`.`updated_at` AS t2_r4,
  `manufacturers`.`id` AS t3_r0,
  `manufacturers`.`name` AS t3_r1,
  `manufacturers`.`code` AS t3_r2,
  `manufacturers`.`created_at` AS t3_r3,
  `manufacturers`.`updated_at` AS t3_r4,
  `categories`.`id` AS t4_r0,
  `categories`.`name` AS t4_r1,
  `categories`.`code` AS t4_r2,
  `categories`.`description` AS t4_r3,
  `categories`.`type` AS t4_r4,
  `categories`.`category_id` AS t4_r5,
  `categories`.`created_at` AS t4_r6,
  `categories`.`updated_at` AS t4_r7,
  `restrictions_products`.`id` AS t5_r0,
  `restrictions_products`.`name` AS t5_r1,
  `restrictions_products`.`code` AS t5_r2,
  `restrictions_products`.`description` AS t5_r3,
  `restrictions_products`.`severity` AS t5_r4,
  `restrictions_products`.`begins_at` AS t5_r5,
  `restrictions_products`.`expires_at` AS t5_r6,
  `restrictions_products`.`type` AS t5_r7,
  `restrictions_products`.`created_at` AS t5_r8,
  `restrictions_products`.`updated_at` AS t5_r9,
  `departments_products`.`id` AS t6_r0,
  `departments_products`.`name` AS t6_r1,
  `departments_products`.`code` AS t6_r2,
  `departments_products`.`description` AS t6_r3,
  `departments_products`.`type` AS t6_r4,
  `departments_products`.`category_id` AS t6_r5,
  `departments_products`.`created_at` AS t6_r6,
  `departments_products`.`updated_at` AS t6_r7,
  `comments`.`id` AS t7_r0,
  `comments`.`title` AS t7_r1,
  `comments`.`body` AS t7_r2,
  `comments`.`author_id` AS t7_r3,
  `comments`.`category` AS t7_r4,
  `comments`.`commentable_id` AS t7_r5,
  `comments`.`commentable_type` AS t7_r6,
  `comments`.`created_at` AS t7_r7,
  `comments`.`updated_at` AS t7_r8,
  `pictures`.`id` AS t8_r0,
  `pictures`.`name` AS t8_r1,
  `pictures`.`title` AS t8_r2,
  `pictures`.`path` AS t8_r3,
  `pictures`.`picturable_id` AS t8_r4,
  `pictures`.`picturable_type` AS t8_r5,
  `pictures`.`category` AS t8_r6,
  `pictures`.`created_at` AS t8_r7,
  `pictures`.`updated_at` AS t8_r8 
FROM
  `products` 
  LEFT OUTER JOIN `restrictions` 
    ON `restrictions`.`id` = `products`.`distribution_restriction_id` 
    AND `restrictions`.`type` IN ('DistributionRestriction') 
  LEFT OUTER JOIN `vendors` 
    ON `vendors`.`id` = `products`.`vendor_id` 
  LEFT OUTER JOIN `manufacturers` 
    ON `manufacturers`.`id` = `products`.`manufacturer_id` 
  LEFT OUTER JOIN `categories` 
    ON `categories`.`id` = `products`.`category_id` 
    AND `categories`.`type` IN ('ProductCategory') 
  LEFT OUTER JOIN `products_restrictions` 
    ON `products_restrictions`.`product_id` = `products`.`id` 
  LEFT OUTER JOIN `restrictions` `restrictions_products` 
    ON `restrictions_products`.`id` = `products_restrictions`.`restriction_id` 
    AND `restrictions_products`.`type` IN ('ProductRestriction') 
  LEFT OUTER JOIN `categories` `departments_products` 
    ON `departments_products`.`id` = `products`.`department_id` 
    AND `departments_products`.`type` IN ('Department') 
  LEFT OUTER JOIN `comments` 
    ON `comments`.`commentable_id` = `products`.`id` 
    AND `comments`.`commentable_type` = 'Product' 
  LEFT OUTER JOIN `pictures` 
    ON `pictures`.`picturable_id` = `products`.`id` 
    AND `pictures`.`picturable_type` = 'Product' 
WHERE `products`.`id` IN (122039) 
ORDER BY `products`.`id` DESC

vs 第二个 Product.last

SQL (2.1 ms) 
SELECT DISTINCT 
  `products`.`id` 
FROM
  `products` 
  LEFT OUTER JOIN `restrictions` 
    ON `restrictions`.`id` = `products`.`distribution_restriction_id` 
    AND `restrictions`.`type` IN ('DistributionRestriction') 
  LEFT OUTER JOIN `vendors` 
    ON `vendors`.`id` = `products`.`vendor_id` 
  LEFT OUTER JOIN `manufacturers` 
    ON `manufacturers`.`id` = `products`.`manufacturer_id` 
  LEFT OUTER JOIN `categories` 
    ON `categories`.`id` = `products`.`category_id` 
    AND `categories`.`type` IN ('ProductCategory') 
  LEFT OUTER JOIN `products_restrictions` 
    ON `products_restrictions`.`product_id` = `products`.`id` 
  LEFT OUTER JOIN `restrictions` `restrictions_products` 
    ON `restrictions_products`.`id` = `products_restrictions`.`restriction_id` 
    AND `restrictions_products`.`type` IN ('ProductRestriction') 
  LEFT OUTER JOIN `categories` `departments_products` 
    ON `departments_products`.`id` = `products`.`department_id` 
    AND `departments_products`.`type` IN ('Department') 
  LEFT OUTER JOIN `comments` 
    ON `comments`.`commentable_id` = `products`.`id` 
    AND `comments`.`commentable_type` = 'Product' 
  LEFT OUTER JOIN `pictures` 
    ON `pictures`.`picturable_id` = `products`.`id` 
    AND `pictures`.`picturable_type` = 'Product' 
LIMIT 1 ;

SELECT 
  `products`.`id` AS t0_r0,
  `products`.`name` AS t0_r1,
  `products`.`client_number` AS t0_r2,
  `products`.`client_reference_string` AS t0_r3,
  `products`.`client_id` AS t0_r4,
  `products`.`vendor_number` AS t0_r5,
  `products`.`vendor_reference_string` AS t0_r6,
  `products`.`vendor_id` AS t0_r7,
  `products`.`manufacturer_number` AS t0_r8,
  `products`.`manufacturer_reference_string` AS t0_r9,
  `products`.`manufacturer_id` AS t0_r10,
  `products`.`reman_number` AS t0_r11,
  `products`.`superseded_by_id` AS t0_r12,
  `products`.`description` AS t0_r13,
  `products`.`category_id` AS t0_r14,
  `products`.`first_year_used` AS t0_r15,
  `products`.`last_year_used` AS t0_r16,
  `products`.`reorder_level` AS t0_r17,
  `products`.`minimum_order_quantity` AS t0_r18,
  `products`.`total_stock` AS t0_r19,
  `products`.`stock_details` AS t0_r20,
  `products`.`status` AS t0_r21,
  `products`.`discontinued_at` AS t0_r22,
  `products`.`currency` AS t0_r23,
  `products`.`dealer_exchange_amount_cents` AS t0_r24,
  `products`.`dealer_sale_amount_cents` AS t0_r25,
  `products`.`retail_exchange_amount_cents` AS t0_r26,
  `products`.`retail_sale_amount_cents` AS t0_r27,
  `products`.`cost_amount_cents` AS t0_r28,
  `products`.`core_charge_amount_cents` AS t0_r29,
  `products`.`reman_amount_cents` AS t0_r30,
  `products`.`out_of_warranty` AS t0_r31,
  `products`.`backordered_till` AS t0_r32,
  `products`.`repair_only` AS t0_r33,
  `products`.`publicly_visible` AS t0_r34,
  `products`.`period_usage` AS t0_r35,
  `products`.`distribution_restriction_id` AS t0_r36,
  `products`.`department_id` AS t0_r37,
  `products`.`stats` AS t0_r38,
  `products`.`announcement_date` AS t0_r39,
  `products`.`status_with_vendor` AS t0_r40,
  `products`.`masterfile_details` AS t0_r41,
  `products`.`created_at` AS t0_r42,
  `products`.`updated_at` AS t0_r43,
  `restrictions`.`id` AS t1_r0,
  `restrictions`.`name` AS t1_r1,
  `restrictions`.`code` AS t1_r2,
  `restrictions`.`description` AS t1_r3,
  `restrictions`.`severity` AS t1_r4,
  `restrictions`.`begins_at` AS t1_r5,
  `restrictions`.`expires_at` AS t1_r6,
  `restrictions`.`type` AS t1_r7,
  `restrictions`.`created_at` AS t1_r8,
  `restrictions`.`updated_at` AS t1_r9,
  `vendors`.`id` AS t2_r0,
  `vendors`.`name` AS t2_r1,
  `vendors`.`code` AS t2_r2,
  `vendors`.`created_at` AS t2_r3,
  `vendors`.`updated_at` AS t2_r4,
  `manufacturers`.`id` AS t3_r0,
  `manufacturers`.`name` AS t3_r1,
  `manufacturers`.`code` AS t3_r2,
  `manufacturers`.`created_at` AS t3_r3,
  `manufacturers`.`updated_at` AS t3_r4,
  `categories`.`id` AS t4_r0,
  `categories`.`name` AS t4_r1,
  `categories`.`code` AS t4_r2,
  `categories`.`description` AS t4_r3,
  `categories`.`type` AS t4_r4,
  `categories`.`category_id` AS t4_r5,
  `categories`.`created_at` AS t4_r6,
  `categories`.`updated_at` AS t4_r7,
  `restrictions_products`.`id` AS t5_r0,
  `restrictions_products`.`name` AS t5_r1,
  `restrictions_products`.`code` AS t5_r2,
  `restrictions_products`.`description` AS t5_r3,
  `restrictions_products`.`severity` AS t5_r4,
  `restrictions_products`.`begins_at` AS t5_r5,
  `restrictions_products`.`expires_at` AS t5_r6,
  `restrictions_products`.`type` AS t5_r7,
  `restrictions_products`.`created_at` AS t5_r8,
  `restrictions_products`.`updated_at` AS t5_r9,
  `departments_products`.`id` AS t6_r0,
  `departments_products`.`name` AS t6_r1,
  `departments_products`.`code` AS t6_r2,
  `departments_products`.`description` AS t6_r3,
  `departments_products`.`type` AS t6_r4,
  `departments_products`.`category_id` AS t6_r5,
  `departments_products`.`created_at` AS t6_r6,
  `departments_products`.`updated_at` AS t6_r7,
  `comments`.`id` AS t7_r0,
  `comments`.`title` AS t7_r1,
  `comments`.`body` AS t7_r2,
  `comments`.`author_id` AS t7_r3,
  `comments`.`category` AS t7_r4,
  `comments`.`commentable_id` AS t7_r5,
  `comments`.`commentable_type` AS t7_r6,
  `comments`.`created_at` AS t7_r7,
  `comments`.`updated_at` AS t7_r8,
  `pictures`.`id` AS t8_r0,
  `pictures`.`name` AS t8_r1,
  `pictures`.`title` AS t8_r2,
  `pictures`.`path` AS t8_r3,
  `pictures`.`picturable_id` AS t8_r4,
  `pictures`.`picturable_type` AS t8_r5,
  `pictures`.`category` AS t8_r6,
  `pictures`.`created_at` AS t8_r7,
  `pictures`.`updated_at` AS t8_r8 
FROM
  `products` 
  LEFT OUTER JOIN `restrictions` 
    ON `restrictions`.`id` = `products`.`distribution_restriction_id` 
    AND `restrictions`.`type` IN ('DistributionRestriction') 
  LEFT OUTER JOIN `vendors` 
    ON `vendors`.`id` = `products`.`vendor_id` 
  LEFT OUTER JOIN `manufacturers` 
    ON `manufacturers`.`id` = `products`.`manufacturer_id` 
  LEFT OUTER JOIN `categories` 
    ON `categories`.`id` = `products`.`category_id` 
    AND `categories`.`type` IN ('ProductCategory') 
  LEFT OUTER JOIN `products_restrictions` 
    ON `products_restrictions`.`product_id` = `products`.`id` 
  LEFT OUTER JOIN `restrictions` `restrictions_products` 
    ON `restrictions_products`.`id` = `products_restrictions`.`restriction_id` 
    AND `restrictions_products`.`type` IN ('ProductRestriction') 
  LEFT OUTER JOIN `categories` `departments_products` 
    ON `departments_products`.`id` = `products`.`department_id` 
    AND `departments_products`.`type` IN ('Department') 
  LEFT OUTER JOIN `comments` 
    ON `comments`.`commentable_id` = `products`.`id` 
    AND `comments`.`commentable_type` = 'Product' 
  LEFT OUTER JOIN `pictures` 
    ON `pictures`.`picturable_id` = `products`.`id` 
    AND `pictures`.`picturable_type` = 'Product' 
WHERE `products`.`id` IN (1)

** 编辑 2**

  default_scope {eager_load([:distribution_restriction, :vendor, :manufacturer, :category, :restrictions, :department, :comments, :pictures])}

【问题讨论】：

请从您的日志中显示每条语句产生的 SQL。
你能发布导致每个结果的 sql 吗？在我的数据库中，它们都导致ORDER BY id DESC 并返回相同的结果。唯一的区别是 last 返回一条记录，last(1) 返回一个数组。
只需运行ActiveRecord::Base.logger = Logger.new(STDOUT)，您就会在控制台中看到日志输出。
产品型号是否有默认订购范围？你在那个模型中覆盖了 self.last 吗？
我认为您的 eager_loading 正在弄乱您的 SQL，您能否再次尝试注释掉 default_scope {eager_load([:distribution_restriction, :vendor, :manufacturer, :category, :restrictions, :department, :comments, :pictures])} 行并将输出粘贴到此处？

标签： mysql ruby-on-rails ruby activerecord

【解决方案1】：

ActiveRecord 的 #first 和 #last 没有任何隐含的顺序，由您的数据库来完成。

在大多数情况下，它们会按主键的顺序返回，但并非总是如此。

最好在您真正需要时明确说明您的订单。

【讨论】：

不完全正确。来自文档：“查找最后一条记录（如果提供了参数，则查找最后 N 条记录）。如果没有定义顺序，它将按主键排序（在这种情况下为 id）。”当我读到它时——对我来说，这意味着它将抓取整个集合（所有行）并根据 id 返回最后一个。我在表中有 30K 条记录，但它一直返回第一条记录 - id 为 1。如果它不应该返回最后一条记录，那么它应该被命名为 random() :-)
基于文档 - 所有东西都相等 last() & last(1) 应该返回相同的东西。 api.rubyonrails.org/classes/ActiveRecord/…

【解决方案2】：

罪魁祸首是我的模型中的 eager_loading。

好的，所以似乎需要牢记这一点，并且可能在文档中提到，eager_loading 会弄乱 .last() 中的排序

我试过了

scope :include_all, -> {eager_load([:distribution_restriction, :vendor, :manufacturer, :category, :restrictions, :department, :comments, :pictures])}

然后 Product.include_all.last & Product.include_all.last(1)

与我最初的问题 eager_loading 一样 - 在第一种情况下排序混乱。

【讨论】：