Rails bug when setting association using same child record, but different child instances

rails4_association_bug.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'rack', github: 'rack/rack'
    gem 'i18n', github: 'svenfuchs/i18n'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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 "comments", :force => true do |t|
    t.integer  "post_id"
    t.string   "text"
  end

  create_table "posts", :force => true do |t|
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
end

class BugTest < ActiveSupport::TestCase
  test "updates association when using same child record, but different child instances" do
    post = Post.create(
      comments: [Comment.create(text: 'old text')]
    )

    comment = Comment.find(post.comments.first.id)
    assert_not_equal post.comments.first.object_id, comment.object_id  # this is required to reproduce bug.

    comment.text = 'new text'
    post.comments = [comment]

    assert_equal 'new text', post.comments.first.text
    # it fails:
    #   1) Failure:
    # BugTest#test_updates_association_when_using_same_child_record,_but_different_child_instances [association_bug.rb:54]:
    # Expected: "new text"
    #   Actual: "old text"
  end
end