This is a Heroku Buildpack for Ruby, Rack, and Rails apps which are not in top-level of the repository. It relies on unchanged Heroku Buildpack for Ruby under the hood.

Let's say you have following repository structure:

.
├── ecommerce
│   ├── crm
│   ├── ordering
│   ├── payments
│   ├── pricing
│   └── product_catalog
└── rails_application
    ├── Gemfile
    ├── Gemfile.lock
    ├── Rakefile
    ├── app
    ├── bin
    ├── config
    ├── config.ru
    ├── db
    ├── lib
    ├── log
    ├── public
    ├── test
    ├── tmp
    └── vendor

There's a Rails application under rails_application/. There's also ecommerce/ — a bunch of components this app relies on, but not the other way round.

Now, you want to promote rails_application/ to Heroku dyno:

• pushing subdirectory via git subtree push --prefix rails_application heroku master would cut you off from the component dependencies

• same issue with other popular monorepo buildpacks, which only promote chosen subdirectory to root

• packaging and distributing components as gems introduces burden when introducing changes and releasing them, quickly defeating benefits of the monorepo

• collapsing everything into a single, top-level package/bundle just for the deployment simply makes production debugging much worse

Luckily with this buildpack, there's no need for compromises!

Tell it where your application lives via APP_DIR variable and it will do the rest:

• preserve exactly the same directory structure on Heroky dyno

• execute stock, up-to-date Ruby buildpack at the application root

• ensure $PATH and $GEM_PATH are aware of given application root

• copy Procfile from application root to top-level directory, making necessary path changes so that Heroku recognizes avaiable process types and can instantly run web

Set it as your only buildpack first:

heroku buildpacks:set https://github.com/arkency/heroku-buildpack-ruby-monorepo

Let it know, where the application is:

heroku config:set APP_DIR=rails_application

• hardcoded Ruby buildpack, there was no need to make it generic so far

After all it is not a very sophisticated Heroku buildpack.