A single file Rails application setting up the DB, a few models, running minitest and finally starting up a Rails server

railsfile_books.rb

require "bundler/inline"

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

  gem "rails"
  gem "pg"
end

require "active_record"
require "action_controller/railtie"

ActiveRecord::Base.establish_connection(adapter: "postgresql", database: "railstestdb")
ActiveRecord::Base.logger = Logger.new(STDOUT)

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

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

  create_table :readings, force: true do |t|
    t.references :book
    t.references :user
    t.timestamps
  end
end

class Book < ActiveRecord::Base
  has_many :readings
  has_many :users, through: :readings

  def self.read_by(*users)
    includes(:readings).where(readings: { user_id: users })
  end
end

class User < ActiveRecord::Base
  has_many :readings
  has_many :books, through: :readings
end

class Reading < ActiveRecord::Base
  belongs_to :book
  belongs_to :user
end

class LibraryApp < Rails::Application
  secrets.secret_token    = "secret_token"
  secrets.secret_key_base = "secret_key_base"

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

  routes.draw do
    resources :books, only: [:index]
  end
end

class BooksController < ActionController::Base
  def index
    @books = Book.all
    render json: @books
  end
end

require "minitest"

class BookTest < Minitest::Test
  def test_read_by
    book1 = Book.create!(name: 'Generation X')
    book2 = Book.create!(name: 'Microserfs')
    book3 = Book.create!(name: 'American Polaroids')

    user1 = User.create!(name: 'Daniel')
    user2 = User.create!(name: 'Susan')
    user3 = User.create!(name: 'Todd')

    Reading.create!(book: book1, user: user1)
    Reading.create!(book: book2, user: user2)
    Reading.create!(book: book2, user: user3)

    books = Book.read_by(user2)
    assert_equal books, [book2, book3]
  end
end

exit_code = Minitest.run
exit exit_code unless exit_code

require "rails/command"
require "rails/commands/server/server_command"

options = {
  app: LibraryApp,
  Host: '0.0.0.0'
}
Rails::Server.new(options).start