Single file Rails

active_record_irb.rb

require 'bundler/inline'

gemfile do
  gem 'activerecord', require: 'active_record'
  gem 'sqlite3', '~> 1.4'
end

ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'
ActiveRecord::Base.logger = Logger.new STDOUT
ActiveRecord::Schema.define do
  create_table :users
  create_table :posts do |t|
    t.references :user, null: false, index: true
    t.string :title, null: false
  end
end

class User < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :user
end

user = User.create!
post = Post.create! user: user, title: "First"
binding.irb

active_record_rspec.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'activerecord', require: 'active_record'
  gem 'sqlite3', '~> 1.4'
  gem 'rspec'
end

ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'
ActiveRecord::Base.logger = Logger.new STDOUT

ActiveRecord::Schema.define do
  create_table :users

  create_table :posts do |t|
    t.references :user, null: false, index: true
    t.string :title, null: false
  end
end

class User < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :user
end

require 'rspec/autorun'

RSpec.describe do
  it "count" do
    User.create!
    expect(User.count).to eq(1)
  end
end

rails_app.ru

# rackup rails_app.ru

require "bundler/inline"

gemfile do
  gem "rack", "~> 2.0"
  gem "rails", "~> 7.1"
  gem "sqlite3", '~> 1.4'
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|
    t.text :content
  end

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

class App < Rails::Application
  config.root = __dir__
  config.load_defaults 7.1
  config.consider_all_requests_local = true
  config.secret_key_base = "i_am_a_secret"
  config.log_level = :debug
  config.logger = Logger.new(STDOUT)
  config.eager_load = false

  routes.append do
    root to: "posts#index"
  end
end

class Post < ActiveRecord::Base
  has_many :comments

  def as_json(options = {})
    super.tap do |json|
      json[:comments] = comments
    end
  end
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

post = Post.create!(content: "Hello, world!")
post.comments.create!(content: "First comment")

class PostsController < ActionController::Base
  def index
    render json: Post.all
  end
end

App.initialize!

run App