Can't Unlink Records in Tables from Ruby Console - ruby-on-rails

Question

Working my way through Lynda.com's course on Rails 5 and I ran into a glitch. Don't know if the issue is my setup, a difference in revs (I believe the instructor is using v5.0.0, while I'm using v5.1.4), I'm doing it wrong, or there's a bug in rails console.
The lesson is Chapter 6 Associations, section 2 One-to-One Associations. The lesson uses two tables: subjects and pages. Their models are:
class Subject < ApplicationRecord
has_one :page
# and a bunch of scopes from an earlier lesson
end
And:
class Page < ApplicationRecord
belongs_to :subject
end
Using MySQL v14.14 Distrib 5.7.19 for osx10.12.
mysql> SHOW FIELDS FROM subjects;
+------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+----------------+
| id | bigint(20) | NO | PRI | NULL | auto_increment |
| name | varchar(50) | YES | | NULL | |
| position | int(11) | YES | | NULL | |
| visible | tinyint(1) | YES | | 0 | |
| created_at | datetime | NO | | NULL | |
| updated_at | datetime | NO | | NULL | |
+------------+-------------+------+-----+---------+----------------+
And:
mysql> SHOW FIELDS FROM pages;
+------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+----------------+
| id | bigint(20) | NO | PRI | NULL | auto_increment |
| subject_id | int(11) | YES | MUL | NULL | |
| name | varchar(50) | YES | | NULL | |
| permalink | varchar(100) | YES | MUL | NULL | |
| position | int(11) | YES | | NULL | |
| visible | tinyint(1) | YES | | 0 | |
| created_at | datetime | NO | | NULL | |
| updated_at | datetime | NO | | NULL | |
+------------+--------------+------+-----+---------+----------------+
The field to watch here is pages.subject_id. Note that it can be nulled.
From rails console I'd created a series of records in subjects, in a previous lesson. In this lesson I create a record in pages and link it with subject record. Great! Works perfectly. Then I try to unlink it just as the instructor does, BUT IT FAILS FOR ME.
At the Rails command line I retrieve a couple of objects from the database:
iirb(main):022:0* subject2 = Subject.find_by_name("Next Subject")
Subject Load (0.6ms) SELECT `subjects`.* FROM `subjects` WHERE `subjects`.`name` = 'Next Subject' LIMIT 1
=> #<Subject id: 2, name: "Next Subject", position: 2, visible: true, created_at: "2017-09-21 21:17:25", updated_at: "2017-09-21 21:51:39">
irb(main):023:0> page2 = Page.find_by_name("Next Page")
Page Load (3.3ms) SELECT `pages`.* FROM `pages` WHERE `pages`.`name` = 'Next Page' LIMIT 1
=> #<Page id: 6, subject_id: nil, name: "Next Page", permalink: "next", position: 2, visible: false, created_at: "2017-09-24 19:00:52", updated_at: "2017-09-24 19:53:38">
Note that pages.subject_id is nil. The records are unlinked.
From the Rails command line both page2.subject and subject2.page return nil, as expected. I can link them with:
irb(main):029:0> subject2.page = page2
(2.7ms) BEGIN
SQL (9.2ms) UPDATE `pages` SET `subject_id` = 2, `updated_at` = '2017-09-25 14:40:09' WHERE `pages`.`id` = 6
(3.0ms) COMMIT
=> #<Page id: 6, subject_id: 2, name: "Next Page", permalink: "next", position: 2, visible: false, created_at: "2017-09-24 19:00:52", updated_at: "2017-09-25 14:40:09">
Now page2.subject and subject2.page works, as expected.
AND THEN THE PROBLEM. The instructor says I can unlink the records with subject2.page = nil, but:
irb(main):037:0* subject2.page = nil
(0.3ms) BEGIN
(0.2ms) ROLLBACK
ActiveRecord::RecordNotSaved: Failed to remove the existing associated page. The record failed to save after its foreign key was set to nil.
from (irb):37
It claims the record failed to save. Could it be the value of nil (or null) in pages.subject_id?
The page2 object shows that subject_id has been reset to its original value, not nil. If I try to set this to nil and then save page2 to the database, it fails.
But if I reload the page2 object in memory from the database, manually set subject_id to nil, and then save page2, it works! At least the pages database shows correct contents.
However if I check page2.subject and subject2.page the Rails command line shows they're still linked. I need to reload these objects in memory from the database before what's in memory shows the correct result.
It seems subject2.page = nil (which works for the instructor) unlinks the records in both the database and memory--at least that's how it's supposed to work. Manually unlinking works in the database, but memory needs to be updated from the database for full functionality.
So what's going on?
Thanks a million for your help.
PS: Mac OS-X v10.12.6 (Sierra), Ruby v2.4.1p111, Rails v5.4.1 (both installed via Homebrew), using VS Code v1.16.1 as my editor, and project setup through the command line (showing exactly the same files and directories as instructor has)
Added info: relevant part of schema.rb
ActiveRecord::Schema.define(version: 20170920222914) do
create_table "pages", force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=utf8" do |t|
t.integer "subject_id"
t.string "name", limit: 50
t.string "permalink", limit: 100
t.integer "position"
t.boolean "visible", default: false
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.index ["permalink"], name: "index_pages_on_permalink"
t.index ["subject_id"], name: "index_pages_on_subject_id"
end
create_table "subjects", force: :cascade, options: "ENGINE=InnoDB DEFAULT CHARSET=utf8" do |t|
t.string "name", limit: 50
t.integer "position"
t.boolean "visible", default: false
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
end

Answer 1

Moving comment down with a bit more detail.
In Rails 5.0 and up they've changed how the default belongs_to associations work. From the upgrade guides it states:
belongs_to will now trigger a validation error by default if the association is not present.
This can be turned off per-association with optional: true.
In your question you show your pages table (thanks for the details by the way) and it indicates that your foreign key can be null. But when you go to save, rails does it's own default validation and rollsback your save since you're using rails 5.1.4.
I believe this is now a default validation as it's helping enforce foreign key constraints. Typically you'd delete the subject and cascade delete the page, or delete the page but keep the subject. But there are times you want to do what you're requesting depending on your structure.
I'm assuming the tutorial your following is using rails 4.2.

Answer 2

It sounds like there's a foreign key constraint in place in your database maybe? Can you update your original post and provide your entire db/schema.rb file please?
If you do not want the Page record to be deleted when you nullify the foreign key, you should specify dependent: :nullify on your has_one association. That may fix your issue too.
class Subject < ApplicationRecord
has_one :page, dependent: :nullify
# and a bunch of scopes from an earlier lesson
end