将ActiveRecord 4.2与MySQL 5.5一起使用
我有一个名为Product的模型。
在控制台中运行Product.last - 我在表格中获得了第一条记录（2013年为id 1和updated_at）
正在运行Product.last(1) - 我在2015年2月的id（122039）和updated_at表中获得了最新记录。
我的印象是他们应该返回相同的。为什么Product.last会返回Product.first的结果？
P.S .:
这是一个开发数据库。所以我将其销毁并重新迁移并重新播种所有数据。不知道它是否重要，但是为了加快播种过程（因为我在一些桌子上播放了10万行的生产级数据） - 我使用批量插入和放大来自.sql文件。 ActiveRecord::Base.connection.execute(sql_from_file)
更新
进行了比较（参见下面的2个编辑），似乎Product.last不执行order by id desc，而Product.last（1）实际执行此操作。有什么想法吗？
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 second 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])}
答案 0 :(得分：1)
ActiveRecord的#first和#last没有任何默示订单，只需要您的数据库即可完成。
在大多数情况下，它们将按主键顺序返回，但并非总是如此。
最好在您真正需要时明确说明您的订单。
答案 1 :(得分：0)
罪魁祸首是我的模型中的eager_loading。
好的，似乎需要记住一些事情，可能会在文档中提到，eager_loading会在.last（）
中排序错误。我试过这个
scope :include_all, -> {eager_load([:distribution_restriction, :vendor, :manufacturer, :category, :restrictions, :department, :comments, :pictures])}
然后
Product.include_all.last＆amp; Product.include_all.last(1)
与我原来的问题一样，eager_loading - 在第一种情况下搞砸了。