rails_single_file.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  warn 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'

  gem 'rails'
  gem 'sqlite3'
  gem 'puma'
end

require 'active_record'
require 'action_controller/railtie'

ActiveRecord::Base.establish_connection(
  adapter: 'sqlite3',
  database: 'test.db' # not :memory:
)
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :books, force: true do |t|
    t.string :name
  end

  create_table :categories, force: true do |t|
    t.string :name
  end

  create_table :categorizations, force: true do |t|
    t.references :book
    t.references :category
    t.boolean :primary, default: false, null: false
  end
end

class Book < ActiveRecord::Base
  has_many :categorizations
  has_many :categories, through: :categorizations
end

class Category < ActiveRecord::Base
  has_many :categorizations
  has_many :books, through: :categorizations

  def self.primaries
    Category.joins(:categorizations).merge(Categorization.primaries)
  end
end

class Categorization < ActiveRecord::Base
  belongs_to :book
  belongs_to :category

  def self.primaries
    where(primary: true)
  end
end

class TestApp < Rails::Application
  secrets.secret_token    = 'secret_token'
  secrets.secret_key_base = 'secret_key_base'

  config.logger = Logger.new($stdout)
  Rails.logger = config.logger

  routes.append do # not routes.draw
    resources :primary_categories, only: :index
  end

  config.consider_all_requests_local = true
end

class PrimaryCategoriesController < ActionController::Base
  include Rails.application.routes.url_helpers

  def index
    @primary_categories = Category.primaries
    render inline: '# of primary categories: <%= @primary_categories.count %>'
  end
end

if __FILE__ == $0
  TestApp.initialize!
  Rack::Server.new(app: TestApp, Port: 3000).start
else
  require 'minitest/autorun'
  class PrimaryCategoriesTest < ActionDispatch::IntegrationTest # :nodoc:
    test 'not works' do
      get '/primary_categories'
      assert_equal 403, status
    end

    private

    def app
      Rails.application
    end
  end
end