Greg Molnar has written a neat article about creating a single-file Rails app Show archive.org snapshot .
This is not meant for production use but can be useful to try things out, e.g. when hunting down a bug or embedding a Rails app into the tests of a gem.

What you do is basically:

Put everything (gems, application config, database migrations, models, controllers) into a single .ru file, like app.ru.
Run it via rackup app.ru. (Hint: if your file is called config.ru, you can just run rackup).
Visit http://localhost:9292/.

There are some caveats to keep in mind:

It will run in whatever Ruby version is active for you. That may be fine locally.
Lock to a specific version by creating a .ruby-version or whatever you need.
Gems and (especially) dependencies will be resolved to their latest version. I recommend creating a Gemfile and moving the gem definition into it so you'll have a Gemfile.lock. Then, run it via
```
bundle exec rackup app.ru
```
You might run into Bundler errors when multiple versions of a gem are installed for your current ruby:
```
You have already activated rack 1.4.7, but your Gemfile requires rack 2.2.4.
```
That can also be resolved by using a separate Gemfile and bundle exec rackup.
Your application is running in the development environment by default. You can use RAILS_ENV=... rackup to change that.

greg.molnar.io

Arne Hartherz

makandra.com

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Over 1 year ago

Henning Koch

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Source code in this card is licensed under the MIT License.

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Posted by Arne Hartherz to makandra dev (2022-08-29 06:04)

Creating a Rails application in a single file

Related cards:

Introduction

Preparation

Understanding sync vs. async control flow

Talking to synchronous (or "blocking") API

Summary