A single-file Rails 5 application.

a_template.html.erb

<!-- NOTE this file must be saved under a folder named "a_subfolder"! -->
<h1>a_template</h1>
<h1><%= @a_variable.an_attribute %></h1>

config.ru

# Based on https://gist.github.com/trevorturk/91cb43c79e71e3d23649f6d103217de7
# To run, cd to directory where this file is saved, and run "rackup".
# Then open http://localhost:9292/apath in your browser.

require 'action_controller/railtie'
require 'active_record'

class AnApp < Rails::Application
  routes.append do
    get "/apath", to: "a#anaction"
  end

  config.secret_key_base = "a" * 30
  config.eager_load = false
  config.logger = Logger.new(STDOUT)
end

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

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

class AModel < ApplicationRecord
  self.table_name = "a_table"
end

class ApplicationController < ActionController::Base
  prepend_view_path "."
end

class AController < ApplicationController
  def anaction
    @a_variable = AModel.first
    render "a_subfolder/a_template"
  end
end

AnApp.initialize!

run AnApp