最近，我遇到了与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'
下一个。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)
最后是错误:
➜ 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">],
[],
[],
[]]
如您所见，集合中仅返回了第一批教师。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)
有人遇到过这样的问题吗？我不明白这是怎么回事。
P.S.如果执行
Resource.all.includes(:teachers).map { |r| r.reload.teachers }，则结果正确。但是，它完全消除了include的含义，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。
最佳答案
我使用ActiveRecord gem在Rails 4.1.6中重新创建了这些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">]]
关于ruby-on-rails - Rails 4 has_and_belongs_to_many在include语句中无法正常工作，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/26507074/