Question

最近我遇到了rails 4和HABTM关系的神秘错误。首先是我的Gemfile：

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

下一步。我的模特：

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

原始数据库数据：

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)

最后的错误：

➜  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">],
[],
[],
[]]

正如您所看到的，只有第一批教师返回收藏中。但是，Rails生成的SQL是正确的并返回所有数据：

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)

以前有人遇到过这样的问题吗？我无法理解这里发生了什么。

P.S。如果您执行Resource.all.includes(:teachers).map { |r| r.reload.teachers }，结果是正确的。然而，它完全消除了include的感觉，并提供了N + 1问题。

更新：还有一个值得一提的发现。如果我删除STI一切正常。

Answer 1

我使用ActiveRecord gem在Rails 4.1.6中重新创建了那些pg模型和数据库记录，并看到了正确的行为：

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">]]

Answer 2

与Rails 4.1.6和pg相同的错误，但我可以通过不删除迁移中的resources_users id字段来获得正确的行为：

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

同样eager_load适用于两种情况（在连接表上有或没有id）并且您可以获得单个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">]]

Rails 4 has_and_belongs_to_many与includes语句无法正常工作

2 个答案: