https://github.com/rails/rails/issues/23265

reproducer.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 'arel', github: 'rails/arel'
  gem 'rack', github: 'rack/rack'
  gem 'sprockets', github: 'rails/sprockets'
  gem 'sprockets-rails', github: 'rails/sprockets-rails'
  gem 'sass-rails', github: 'rails/sass-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|
    t.integer :comments_count, :default => 0
  end

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

class Post < ActiveRecord::Base  #domain
  has_many :posts
end

class Comment < ActiveRecord::Base  #hg
  belongs_to :post, :counter_cache => :comments_count
end

class BugTest < Minitest::Test
  def test_counter_update
    post1, post2 = Post.create!, Post.create!

    comment1 = Comment.create!(:post => post1)
    puts "Created comment #{comment1.id} against post #{post1.id}"
    assert_equal 1, post1.reload.comments_count
    assert_equal 0, post2.reload.comments_count
    puts "Asserted comment counters"

    # Remove this finder to prevent a reload, which exposes the bug.  Using the same
    # newly created object prevents the counter being incremented again.
    comment1 = Comment.find_by_id(comment1.id)

    # The counter of both models are updated twice in this call instead of once.
    #
    # It's the model's association setter that's incrementing the counter, as well as
    # a model callback monitoring post_id for changes.
    comment1.update_attribute(:post, post2)
    puts "Updated comment #{comment1.id} against post #{post2.id}"

    assert_equal 0, post1.reload.comments_count
    assert_equal 1, post2.reload.comments_count
  end
end