(大体)Single file Rails Application - https://github.com/suburi/simple-app

config.ru

require './mini_app'

run MiniApp::Application

Gemfile

source "https://rubygems.org"

gem "rails"
gem "sqlite3"

mini_app.rb

require 'active_record'
require 'action_controller/railtie'

ActiveRecord::Base.establish_connection(:adapter => 'sqlite3', :database => ':memory:')

module MiniApp
  class Application < Rails::Application
    config.secret_token = [*'A'..'z'].join
    config.session_store :cookie_store, :key => '_miniapp_session'
    config.eager_load = false
  end
end

MiniApp::Application.initialize!

module ApplicationHelper; end

class ApplicationController < ActionController::Base
end

class BooksController < ApplicationController
  def index
    @books = Book.all
    render :inline => <<-ERB
<%= 'Hello World' %>
ERB
  end
end

MiniApp::Application.routes.draw do
  root 'books#index'
  resources :books
end

class Book < ActiveRecord::Base
end

class CreateAllTables < ActiveRecord::Migration
  def self.up
    create_table :books do |t|
      t.string :title
      t.integer :price
    end

    Book.create title: "Alice's Adventures in Wonderland", price: 500
  end
end

CreateAllTables.up unless ActiveRecord::Base.connection.table_exists? 'books'

run.sh

#!/bin/bash
bundle exec rackup -p 3000 ./config.ru