executable.rb

require "bundler/inline"

gemfile do
  gem "rails"
  gem "sqlite3"
end

require "active_record"
require "action_controller"
require "rails"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :posts do |t|
    t.references :user, null: false, index: true
    t.string :title, null: false
    t.text :content, null: false
  end

  create_table :users do |t|
  end
end

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class Post < ApplicationRecord
  belongs_to :user
end

class User < ApplicationRecord
  has_many :posts
end

user = User.create!
post = Post.create! user:, title: "First", content: "Heyo"

class UsersController < ActionController::Base
  def show
    @user = User.first
    render json: @user
  end
end

class App < Rails::Application
  routes.default_url_options[:host] = 'localhost:3000'

  config.logger = Logger.new(STDOUT)

  routes.append do
    resources :users
  end
end

App.initialize!

require "rackup"
Rackup::Server.start(app: Rails.application, Port: 3000)

# binding.irb