Open up the provided Nix shell:

nix develop --ignore-environment

Alternatively, you can run direnv allow to activate the shell in your current environment.

Then you can run the Nix-bundled Rails app by just running rails. The standard ./bin/rails also works.

To use the Rails environment without checking out this repo:

nix develop 'github:the-nix-way/nix-flake-dev-environments?dir=ruby-on-rails'

Note that this environment only works in conjunction with an existing Gemfile, Gemfile.lock, and gemset.nix. If you have a Gemfile.lock but no gemset.nix:

nix run nixpkgs#bundix

To make updates to the app's dependencies, such as bumping the Rails version, there's a special script that generates a new Gemfile.lock and gemset.nix:

update-deps

Once the new Gemfile.lock and gemset.nix have been created, then opening up the default shell using nix develop should work as expected but with the new dependencies.