ActiveRecord.last和ActiveRecord.last（1）返回不同结果

问题描述：

使用ActiveRecord 4.2与MySQL 5.5ActiveRecord.last和ActiveRecord.last（1）返回不同结果

我有一个名为Product的模型。

在控制台中运行Product.last - 我得到的第一个记录表（ID为1和的updated_at 2013年）

运行Product.last(1) - 我得到的最后一个记录在表ID（122039）和的updated_at 2月

我以为他们应该返回相同的印象。为什么Product.last会返回Product.first的结果？

P.S .: 这是一个开发数据库。所以我销毁了它并重新迁移它并重新播种了所有数据。不知道它的问题，而是要加快播种进程（因为我播种一些表与生产级的数据行的100,000s） - 我使用批量插入& ActiveRecord::Base.connection.execute(sql_from_file)

UPDATE从.sql文件种子

做了一个比较（见下面的2个编辑），似乎Product.last没有做order by id desc，而Product.last（1）实际上做了。任何想法为什么？

EDIT 1 从原木：

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])}

请显示每条语句生成的日志中的SQL。 – smathy 2015-02-23 16:43:24

你可以发布在每个这些结果的SQL？在我的数据库中，他们都会导致'ORDER BY ID DESC'并返回相同的结果。唯一的区别是'last'返回一个记录，'last（1）'返回一个数组。 – evanbikes 2015-02-23 16:43:25

只需运行'ActiveRecord :: Base.logger = Logger.new（STDOUT）'，你就会在控制台中看到日志输出。 – smathy 2015-02-23 17:10:57

答

罪魁祸首就是我的模型中的eager_loading。

好了，所以它似乎这件事情需要牢记，并在文档也许提到，eager_loading弄乱了排序中。去年（）

我想这

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

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

与我原来的问题eager_loading - 在fi中搞乱顺序第一种情况。

答

ActiveRecord的#first和#last不具有任何隐含的排序，而是由您的数据库执行。

在大多数情况下，它们将按主键顺序返回，但并非总是如此。

当你真的需要它时，最好明确你的订单。

不完全正确。来自docs：“查找最后一条记录（如果提供了参数，则记录最后N条记录）如果没有定义顺序，它将按主键排序（本例中为id）。” 当我读它 - 对我来说，这意味着它会抓住整个集合（所有行），并返回基于id的最后一个。我在表格中有30K条记录，但它会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（）:-) – konung 2015-02-28 01:34:20

基于文档 - 所有的东西等于last（）和last（1）应该返回相同的结果。 http://api.rubyonrails.org/classes/ActiveRecord/FinderMethods.html#method-i-last – konung 2015-02-28 01:39:03

ActiveRecord.last和ActiveRecord.last（1）返回不同结果

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