test.rb

unless File.exists?('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'
 
# 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 do |t|
  end
 
  create_table :categories do |t|
  end

  create_table :categories_posts, :id => false do |t|
    t.integer :category_id,   :null => false
    t.integer :post_id, :null => false
  end
end
 
class Post < ActiveRecord::Base
  has_and_belongs_to_many :categories
end
 
class Category < ActiveRecord::Base
  has_and_belongs_to_many :posts
end

class TestIt < Minitest::Test
  def test_it
    p = Post.create
    c1 = Category.create
    c2 = Category.create
    p.categories << c1
    p.categories << c2

    posts = Post.joins(:categories).where("categories_posts.category_id != ?", 1)
 
    assert_equal [], posts
  end
end