Self executable gist

association_ids.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    #gem 'rails', github: 'rails/rails'
    gem 'rails', github: 'rails/rails', branch: '4-2-stable'
    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 :books do |t|
    t.string :name
  end

  create_table :categories do |t|
    t.string :name
  end

  create_table :books_categories do |t|
    t.integer :book_id
    t.integer :category_id
  end
end

class Book < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

class Category < ActiveRecord::Base
  has_and_belongs_to_many :books
end

class Test < Minitest::Test
  def test_association_ids
    book = Book.create!
    category = Category.create!
    book.category_ids = [category.id]

    assert_equal book.category_ids, [category.id]
    assert_raises(ActiveRecord::RecordNotFound) { book.category_ids += [category.id] } # ?!
  end
end