Outdated gist related to https://github.com/rails/rails/issues/13304. Move along.

counter_cache_test.rb

require 'rubygems'
require 'sqlite3'
require 'active_record'
require 'minitest/autorun'

ActiveRecord::Base.establish_connection 'sqlite3:///:memory:'

ActiveRecord::Schema.define do
  create_table(:posts) do |t|
    t.integer :author_count, default: 0
  end
  create_table(:authors)
  create_table(:authorships) do |t|
    t.integer :author_id
    t.integer :post_id
  end
end

# Post has many Authors through Authorships
class Post < ActiveRecord::Base
  has_many :authorships
  has_many :authors,
           through: :authorships
end

# Joining model between Author <=> Post
class Authorship <  ActiveRecord::Base
  belongs_to :author
  belongs_to :post, counter_cache: :author_count
end

# Author has many Posts
class Author < ActiveRecord::Base
  has_many :authorships
  has_many :posts, through: :authorships
end

# Replicate the counter_cache decriment issue when removing an Author from Post
class BugTest < Minitest::Test
  def test_counter_cache
    post = Post.create!
    author_1 = Author.create!
    author_2 = Author.create!

    assert_equal 0, post.author_count

    post.authors << author_1
    assert_equal 1, post.reload.author_count

    post.authors << author_2
    assert_equal 2, post.reload.author_count

    post.authors.destroy(author_2)
    assert_equal 1, post.reload.author_count
  end
end

Gemfile

source 'https://rubygems.org'
gem 'activerecord', github: 'rails/rails'
gem 'sqlite3'
gem 'minitest', '~> 5.1'

Gemfile.lock

GIT
  remote: git://github.com/rails/rails.git
  revision: 1a6089f30565cc3fd19c02c2331fdd255ff23502
  specs:
    activemodel (4.1.0.beta1)
      activesupport (= 4.1.0.beta1)
      builder (~> 3.1)
    activerecord (4.1.0.beta1)
      activemodel (= 4.1.0.beta1)
      activesupport (= 4.1.0.beta1)
      arel (~> 5.0.0)
    activesupport (4.1.0.beta1)
      i18n (~> 0.6, >= 0.6.9)
      json (~> 1.7, >= 1.7.7)
      minitest (~> 5.1)
      thread_safe (~> 0.1)
      tzinfo (~> 1.1)

GEM
  remote: https://rubygems.org/
  specs:
    arel (5.0.0)
    atomic (1.1.14)
    builder (3.2.2)
    i18n (0.6.9)
    json (1.8.1)
    minitest (5.2.0)
    sqlite3 (1.3.8)
    thread_safe (0.1.3)
      atomic
    tzinfo (1.1.0)
      thread_safe (~> 0.1)

PLATFORMS
  ruby

DEPENDENCIES
  activerecord!
  minitest (~> 5.1)
  sqlite3

-- create_table(:posts)
-> 0.0208s
-- create_table(:authors)
-> 0.0003s
-- create_table(:authorships)
-> 0.0004s
Run options: --seed 65434

Running:

.

Finished in 0.024955s, 40.0721 runs/s, 160.2885 assertions/s.

1 runs, 4 assertions, 0 failures, 0 errors, 0 skips