Incorrectly raising ReadonlyAttributeError when autosaving has_one association with readonly foreign key

[Earlopain]

Rails 7.1 sets raise_on_assign_to_attr_readonly to true. In the following repro I set a column unrelated to attr_readonly yet the error is still being set. If I stop setting this column the error goes away.

Steps to reproduce

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails", branch: "main"

  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord.raise_on_assign_to_attr_readonly = true

ActiveRecord::Schema.define do
  create_table :forum_topics, force: true do |t|
    t.integer :updater_id
    t.text :title
  end

  create_table :forum_posts, force: true do |t|
    t.integer :updater_id
    t.integer :forum_topic_id
    t.text :body
  end
end

class ForumTopic < ActiveRecord::Base
  has_one :original_forum_post, -> { order("forum_posts.id asc") }, class_name: "ForumPost", inverse_of: :forum_topic
  validates_associated :original_forum_post
  accepts_nested_attributes_for :original_forum_post
end

class ForumPost < ActiveRecord::Base
  attr_readonly :forum_topic_id
  belongs_to :forum_topic

  before_validation do |rec|
    # next # Skipping setting this column makes the error go away
    if persisted?
      rec.updater_id = 456
    else
      rec.updater_id = 123
    end
  end
end

class BugTest < Minitest::Test
  def test_readonly_attr_exception
    topic = ForumTopic.create!(title: "Hi!", original_forum_post_attributes: { body: "What's up?" })
    topic.update(title: "nevermind") # => ActiveRecord::ReadonlyAttributeError: forum_topic_id
    assert_equal(456, topic.original_forum_post.updater_id)
  end
end

System configuration

Rails version: master

Ruby version: 3.2.2

[joshuay03]

This is an interesting one! It looks like because updater_id on the nested record is changing, an autosave is triggered (this is always enabled for nested attributes), causing the foreign key on the nested record to be set here despite it not changing:

rails/activerecord/lib/active_record/autosave_association.rb

Line 461 in 351a8d9

record[foreign_key] = _read_attribute(primary_key)

Interestingly, this isn't an issue on belongs_to which already only sets the nested record's foreign key if it's changing:

rails/activerecord/lib/active_record/autosave_association.rb

Line 524 in 351a8d9

self[foreign_key] = association_id unless self[foreign_key] == association_id

I've got a fix passing against your script, just adding some coverage before I get a PR up.

[joshuay03]

Got this up: #50901

[Earlopain]

Thank you, that looks good and gets all my actual tests passing. This seems to be mirroring what #46759 did for belongs_to, I found #46758 while searching for similar issues but didn't connect the dots.

[joshuay03]

Thank you, that looks good and gets all my actual tests passing. This seems to be mirroring what #46759 did for belongs_to, I found #46758 while searching for similar issues but didn't connect the dots.

Ah nice find, I'll reference it in my PR, thanks!