Eager loaded belongs_to associations are always readonly

active_record_master.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', github: 'rails/rails'
  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::Schema.define do
  create_table :posts, force: true  do |t|
  end

  create_table :comments, force: true  do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_join_doesnt_make_readonly
    post = Post.create!
    post.comments << Comment.create!

    comment = Comment.where(id: "1").joins(:post).first!

    assert !comment.post.readonly?
  end

  def test_eager_load_doesnt_make_readonly
    post = Post.create!
    post.comments << Comment.create!

    comment = Comment.where(id: "1").eager_load(:post).first!

    assert !comment.post.readonly?
  end
end

=begin
Output:

  1) Failure:
BugTest#test_eager_load_doesnt_make_readonly [active_record_master.rb:55]:
Failed assertion, no message given.

2 runs, 2 assertions, 1 failures, 0 errors, 0 skips
=end