Bootstrapping Rails on NixOS

recently tried to start a Rails project on NixOS, realized that there was no obvious way to start a new one, even though it seemed fairly straightforward to work on an existing project.

i figured out a fairly simple process. i use direnv and flakes, and this process does too.

first, create a project directory. enter it. create the following two files to set up a trivial flake:

{
    description = "TODO";

    inputs.flake-utils.url = github:numtide/flake-utils;

    outputs = { self, nixpkgs, flake-utils }:
        flake-utils.lib.eachDefaultSystem (system: let
            pkgs = nixpkgs.legacyPackages.${system};
        in {
            packages.default = pkgs.callPackage ./default.nix;
        });
}

{ stdenv, ruby_3_1 }:

stdenv.mkDerivation {
    pname = "TODO";
    version = "TODO";

    nativeBuildInputs = [];
    buildInputs = [ ruby_3_1 ];
}

then, create an .envrc with the following content:

use flake
layout ruby

run direnv allow.

next, write a bootstrap Gemfile:

source 'https://rubygems.org'

gem 'rails'

install locally by running bundle install --path=vendor (ignore the warnings for now)

finally, run bundle exec rails install . and, when asked, overwrite the bootstrap gemfile with the upstream default.