Rails 4 has_and_belongs_to_many无法正常使用包含语句

最近我遇到了与轨道4和HABTM关系的神秘错误。首先我的Gemfile：Rails 4 has_and_belongs_to_many无法正常使用包含语句

source 'https://rubygems.org' 
gem 'rails', '~> 4.1.6' 
gem 'pg'

下一页。我的模型：

class User < ActiveRecord::Base 
end 

class Teacher < User 
    has_and_belongs_to_many :resources, foreign_key: :user_id 
end 

class Resource < ActiveRecord::Base 
    has_and_belongs_to_many :teachers, association_foreign_key: :user_id 
end

原始DB数据：

select * from resources; 
id |   created_at   |   updated_at 
----+----------------------------+---------------------------- 
    1 | 2014-10-13 08:24:07.308361 | 2014-10-13 08:24:07.308361 
    2 | 2014-10-13 08:24:07.889907 | 2014-10-13 08:24:08.156898 
    3 | 2014-10-13 08:24:08.68579 | 2014-10-13 08:24:08.884731 
    4 | 2014-10-13 08:24:09.997244 | 2014-10-13 08:24:10.205753 
(4 rows) 

select * from users; 
id |   created_at   |   updated_at   | type 
----+----------------------------+----------------------------+--------- 
13 | 2014-10-13 08:24:01.086192 | 2014-10-13 08:24:01.086192 | Teacher 
12 | 2014-10-13 08:24:00.984957 | 2014-10-13 08:24:00.984957 | Teacher 
    2 | 2014-10-13 08:23:59.950349 | 2014-10-16 08:46:02.531245 | Teacher 
(3 rows) 

select * from resources_users; 
user_id | resource_id 
---------+------------- 
     13 |   1 
     2 |   2 
     12 |   3 
     2 |   4 
(4 rows)

最后的bug：

➜ rails_test bundle exec rails c 
Loading development environment (Rails 4.1.6) 
2.1.2 :001 > Resource.all.includes(:teachers).map(&:teachers).map(&:to_a) 
    Resource Load (0.6ms) SELECT "resources".* FROM "resources" 
    SQL (1.3ms) SELECT "resources_users".*, "resources_users"."user_id" AS t0_r0, "resources_users"."resource_id" AS t0_r1, "users"."id" AS t1_r0, "users"."created_at" AS t1_r1, "users"."updated_at" AS t1_r2, "users"."type" AS t1_r3 FROM "resources_users" LEFT OUTER JOIN "users" ON "users"."id" = "resources_users"."user_id" AND "users"."type" IN ('Teacher') WHERE "users"."type" IN ('Teacher') AND "resources_users"."resource_id" IN (1, 2, 3, 4) 
=> [ 
[#<Teacher id: 13, created_at: "2014-10-13 08:24:01", updated_at: "2014-10-13 08:24:01", type: "Teacher">], 
[], 
[], 
[]]

正如你看到的教师只有第一阵列中收集返回。然而，通过Rails的生成的SQL是正确的，返回的所有数据：

SELECT "resources_users".*, "resources_users"."user_id" AS t0_r0, "resources_users"."resource_id" AS t0_r1, "users"."id" AS t1_r0, "users"."created_at" AS t1_r1, "users"."updated_at" AS t1_r2, "users"."type" AS t1_r3 FROM "resources_users" LEFT OUTER JOIN "users" ON "users"."id" = "resources_users"."user_id" AND "users"."type" IN ('Teacher') WHERE "users"."type" IN ('Teacher') AND "resources_users"."resource_id" IN (1, 2, 3, 4); 
user_id | resource_id | t0_r0 | t0_r1 | t1_r0 |   t1_r1   |   t1_r2   | t1_r3 
---------+-------------+-------+-------+-------+----------------------------+----------------------------+--------- 
     13 |   1 | 13 |  1 | 13 | 2014-10-13 08:24:01.086192 | 2014-10-13 08:24:01.086192 | Teacher 
     2 |   2 |  2 |  2 |  2 | 2014-10-13 08:23:59.950349 | 2014-10-16 08:46:02.531245 | Teacher 
     12 |   3 | 12 |  3 | 12 | 2014-10-13 08:24:00.984957 | 2014-10-13 08:24:00.984957 | Teacher 
     2 |   4 |  2 |  4 |  2 | 2014-10-13 08:23:59.950349 | 2014-10-16 08:46:02.531245 | Teacher 
(4 rows)

这儿还有没有人之前遇到过这样的问题？我无法理解这里发生了什么。

P.S.如果你做Resource.all.includes(:teachers).map { |r| r.reload.teachers }结果是正确的。然而，它从根本上消除了include的意义，并提供了N + 1问题。

更新：另外一个发现值得一提。如果我删除STI，一切正常。

来源

2014-10-22 Pavel S

只是想在这里指出'has_and_belongs_to_many'是排序的用于多对多关系的老式学校。更好的选择是'has_many：through'。 [来源]（http://aihuiong.com/post/829841945/rails-hasandbelongstomany-vs-hasmany-through） – zhurora 2015-10-31 17:44:20

只想提一下，关于'has_and_belongs_to_many'没有什么“老派”，你应该使用这种关联类型是最适合你的任务的。对于很多事情来说，HABTM是完全没问题的。 – sevenseacat 2015-11-05 07:09:15

我重新用Rails 4.1.6那些ActiveRecord模型和数据库中的记录与pg宝石，看到正确的行为：

irb(main):017:0> Resource.all.includes(:teachers).map(&:teachers).map(&:to_a) Resource Load (0.6ms) SELECT "resources".* FROM "resources" SQL (6.9ms) SELECT "resources_users".*, "resources_users"."id" AS t0_r0, "resources_users"."resource_id" AS t0_r1, "resources_users"."user_id" AS t0_r2, "users"."id" AS t1_r0, "users"."type" AS t1_r1, "users"."created_at" AS t1_r2, "users"."updated_at" AS t1_r3 FROM "resources_users" LEFT OUTER JOIN "users" ON "users"."id" = "resources_users"."user_id" AND "users"."type" IN ('Teacher') WHERE "users"."type" IN ('Teacher') AND "resources_users"."resource_id" IN (1, 2, 3, 4) => [[#<Teacher id: 13, type: "Teacher", created_at: "2015-11-05 07:02:59", updated_at: "2015-11-05 07:02:59">], [#<Teacher id: 2, type: "Teacher", created_at: "2015-11-05 07:02:20", updated_at: "2015-11-05 07:02:32">], [#<Teacher id: 12, type: "Teacher", created_at: "2015-11-05 07:03:50", updated_at: "2015-11-05 07:03:50">], [#<Teacher id: 2, type: "Teacher", created_at: "2015-11-05 07:02:20", updated_at: "2015-11-05 07:02:32">]]

来源

2015-11-05 07:06:44

同样的bug这里使用Rails 4.1.6和pg，但我可以得到正确的行为在迁移不删除resources_usersid领域：

def change 
    #create_table :resources_users, id: false do |t| 
    create_table :resources_users do |t| 
    t.integer :resource_id 
    t.integer :user_id 
    end 
end

而且eager_load作品在b超视距情况下（有或没有id在连接表），你有一个SQL查询的好处：

Resource.eager_load(:teachers).map(&:teachers).map(&:to_a)

输出：

irb(main):002:0> Resource.eager_load(:teachers).map(&:teachers).map(&:to_a) 
SQL (1.0ms) SELECT "resources"."id" AS t0_r0, "resources"."created_at" 
AS t0_r1, "resources"."updated_at" AS t0_r2, "users"."id" AS t1_r0, 
"users"."type" AS t1_r1, "users"."created_at" AS t1_r2, 
"users"."updated_at" AS t1_r3 FROM "resources" LEFT OUTER JOIN 
"resources_users" ON "resources_users"."resource_id" = "resources"."id" 
LEFT OUTER JOIN "users" ON "users"."id" = "resources_users"."user_id" 
AND "users"."type" IN ('Teacher') 
=> [[#<Teacher id: 1, type: "Teacher", created_at: "2015-11-05 
15:02:33", updated_at: "2015-11-05 15:02:33">], [#<Teacher id: 2, type: 
"Teacher", created_at: "2015-11-05 15:02:33", updated_at: "2015-11-05 
15:02:33">], [#<Teacher id: 2, type: "Teacher", created_at: "2015-11-05 
15:02:33", updated_at: "2015-11-05 15:02:33">], [#<Teacher id: 3, type: 
"Teacher", created_at: "2015-11-05 15:02:33", updated_at: "2015-11-05 
15:02:33">]]

来源

2015-11-05 15:23:46 ehoffmann

Rails 4 has_and_belongs_to_many无法正常使用包含语句

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