A Single file Rails Application

09 Aug 2022
#ruby #rails

Did you know that you can create a single-file Rails application?

You shouldn't unless you are experimenting with something, but regardless, let me show you how it's done.

Let's create a file and call it "app.ru", the "ru" extension means it will be a rackup file. Then we need to use bundler inline to install the dependencies: 

# frozen_string_literal: true
require 'bundler/inline'

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

  gem 'rails', '~> 7.0.4'
  gem 'sqlite3'
end

Next, we require Rails and configure Active Record to use a sqlite database and we define the database schema and create a model: 

# frozen_string_literal: true
require 'bundler/inline'

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

  gem 'rails', '~> 7.0.4'
  gem 'sqlite3'
end

require 'rails/all'
database = 'development.sqlite3'

ENV['DATABASE_URL'] = "sqlite3:#{database}"
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: database)
ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

Next step is to create an "App" class, and set the necessary configuration. We also set the root route to point to the WelcomeController's index action:

# frozen_string_literal: true
require 'bundler/inline'

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

  gem 'rails', '~> 7.0.4'
  gem 'sqlite3'
end

require 'rails/all'
database = 'development.sqlite3'

ENV['DATABASE_URL'] = "sqlite3:#{database}"
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: database)
ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

class App < Rails::Application
  config.root = __dir__
  config.consider_all_requests_local = true
  config.secret_key_base = 'i_am_a_secret'
  config.active_storage.service_configurations = { 'local' => { 'service' => 'Disk', 'root' => './storage' } }

  routes.append do
    root to: 'welcome#index'
  end
end
Final step is to add a controller, initialize the app and run it. Then from a terminal, you can run "rackup app.ru" and you have a single file Rails application. 
# frozen_string_literal: true
require 'bundler/inline'

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

  gem 'rails', '~> 7.0.4'
  gem 'sqlite3'
end

require 'rails/all'
database = 'development.sqlite3'

ENV['DATABASE_URL'] = "sqlite3:#{database}"
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: database)
ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

class App < Rails::Application
  config.root = __dir__
  config.consider_all_requests_local = true
  config.secret_key_base = 'i_am_a_secret'
  config.active_storage.service_configurations = { 'local' => { 'service' => 'Disk', 'root' => './storage' } }

  routes.append do
    root to: 'welcome#index'
  end
end

class WelcomeController < ActionController::Base
  def index
    render inline: 'Hi!'
  end
end

App.initialize!

run App

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