【发布时间】：2014-12-17 21:08:05
【问题描述】：
最近我遇到了一个关于 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'
接下来。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)
终于出了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">],
[],
[],
[]]
如您所见，集合中只返回了第一组教师。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)
以前有人遇到过这样的问题吗？我不明白这里发生了什么。
附：如果你这样做Resource.all.includes(:teachers).map { |r| r.reload.teachers }，结果是正确的。然而，它完全消除了include 的意义，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。
【问题讨论】：
-
只想在这里提一下
has_and_belongs_to_many是一种老式的多对多关系方式。更好的选择是has_many :through。 Source -
只想提一下
has_and_belongs_to_many没有什么“老派”，您应该使用最适合您任务的关联类型。在很多方面，HABTM 都很好。
标签： ruby-on-rails postgresql ruby-on-rails-4 rails-activerecord has-and-belongs-to-many