https://github.com/rails/rails/issues/14870

rails4-1-unexpected-behavior2.rb

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

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :articles

  create_table :taggings do |t|
    t.belongs_to :article
    t.belongs_to :tag
  end
  create_table :tags

  create_table :article_categories do |t|
    t.belongs_to :article
    t.belongs_to :category
  end
  create_table :categories
end

class Article < ActiveRecord::Base
  has_many :taggings
  has_many :tags, through: :taggings

  has_many :article_categories
  has_many :categories, through: :article_categories
end

class Tagging < ActiveRecord::Base
  belongs_to :article
  belongs_to :tag

  after_create do
    article.reload
    puts article.tags.count
  end
end

class Tag < ActiveRecord::Base
  has_many :taggings
  has_many :articles, through: :taggings
end

class ArticleCategory < ActiveRecord::Base
  belongs_to :article
  belongs_to :category
end

class Category < ActiveRecord::Base
  has_many :article_categories
  has_many :articles, through: :article_categories
end

class BugTest < Minitest::Test
  def test_create_with_has_many_through_association
    tag = Tag.create
    category = Category.create
    article = Article.create(tags: [tag], categories: [category])
    p ArticleCategory.count  # => 0
  end
end