https://github.com/rails/rails/issues/18273-rails 4.2.0

gistfile1.rb

# https://github.com/rails/rails/issues/18273

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.2.0'
require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

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

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
    t.string :bar
  end
end

class Post < ActiveRecord::Base
  def change_bar
    attribute_will_change!(:bar)
    self.bar = "bar"
  end
  def barbar
    self.bar = "barbar"
  end
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    post.change_bar
    post.barbar
    post.save!

    assert_equal 'barbar', post.bar
  end
end

# $ ruby issue-18273.rb
# -- create_table(:posts, {:force=>true})
# D, [2015-01-04T15:57:45.355569 #75381] DEBUG -- :    (3.4ms)  DROP TABLE "posts"
# D, [2015-01-04T15:57:45.361558 #75381] DEBUG -- :    (5.7ms)  CREATE TABLE "posts" ("id" serial primary key, "bar" character varying)
#    -> 0.0294s
# Run options: --seed 25999

# # Running:

# D, [2015-01-04T15:57:45.409134 #75381] DEBUG -- :    (0.1ms)  BEGIN
# D, [2015-01-04T15:57:45.412621 #75381] DEBUG -- :   SQL (0.7ms)  INSERT INTO "posts" DEFAULT VALUES RETURNING "id"
# D, [2015-01-04T15:57:45.413375 #75381] DEBUG -- :    (0.3ms)  COMMIT
# D, [2015-01-04T15:57:45.413674 #75381] DEBUG -- :    (0.1ms)  BEGIN
# D, [2015-01-04T15:57:45.418277 #75381] DEBUG -- :   SQL (0.3ms)  UPDATE "posts" SET "bar" = $1 WHERE "posts"."id" = $2  [["bar", "barbar"], ["id", 1]]
# D, [2015-01-04T15:57:45.418717 #75381] DEBUG -- :    (0.3ms)  COMMIT
# .

# Finished in 0.019245s, 51.9615 runs/s, 51.9615 assertions/s.

# 1 runs, 1 assertions, 0 failures, 0 errors, 0 skips