Rails Preview Demo

A minimal Rails application for verifying preview deployments on Railway and Fly.io.

Tech Stack

Layer	Technology	Version
Language	Ruby	3.4.8
Framework	Rails	8.1.2
Database	PostgreSQL	17
Web Server	Puma	7.2.0
Assets	Propshaft + Importmap	1.3.1 / 2.2.3

Features

• Post CRUD — scaffold with title (string), body (text), and published (boolean)
• Health Check — GET /up returns 200 if the app boots successfully
• Dockerfile — multi-stage production build generated by Rails 8.1

Routes

Method	Path	Description
GET	/	Root (redirects to posts#index)
GET	/up	Health check
GET	/posts	List all posts
GET	/posts/new	New post form
GET	/posts/:id	Show a post
GET	/posts/:id/edit	Edit post form
POST	/posts	Create a post
PATCH/PUT	/posts/:id	Update a post
DELETE	/posts/:id	Delete a post

Local Development

Prerequisites

• Ruby 3.4.8 (managed via asdf)
• PostgreSQL 17
• Bundler

Setup

bundle install
bin/rails db:create db:migrate
bin/rails server

Open http://localhost:3000

Deployment

Railway

Deployment is configured via railway.json.

Configuration

Setting	Value	Description
Builder	Dockerfile	Uses the multi-stage Dockerfile generated by Rails
Health check	GET /up	Rails built-in health endpoint (timeout: 300s)
Restart policy	On failure (max 5 retries)	Prevents infinite crash loops

The Dockerfile's entrypoint automatically runs db:prepare before starting the server. No custom start command is needed.

Dashboard Setup

1. Create a new project and connect this repository
2. Add a PostgreSQL service
3. On the app service, add a variable: DATABASE_URL = ${{Postgres.DATABASE_URL}}
4. On the app service, add a variable: RAILS_MASTER_KEY (value from config/master.key)
5. Enable PR environments: Project Settings > Environments > Enable PR Environments
6. Enable bot PR environments: Project Settings > Environments > Enable Bot PR Environments

Note: If you use AI coding tools like Claude Code to create commits and pull requests, you must enable Bot PR Environments. Railway classifies PRs with commits from supported bots (including Claude Code) as bot PRs, and will not create preview environments for them unless this setting is turned on.

PR Preview Environments

When PR environments are enabled, Railway automatically:

• Creates an isolated environment (including a fresh PostgreSQL instance) for each pull request
• Provisions a unique URL (e.g., *.up.railway.app) for the PR environment
• Runs db:prepare on the empty database via the Dockerfile entrypoint
• Deletes the environment when the PR is merged or closed

Fly.io

Deployment is configured via fly.toml. PR preview environments are handled by GitHub Actions using superfly/fly-pr-review-apps.

Configuration

Setting	Value	Description
Builder	Dockerfile	Uses the multi-stage Dockerfile generated by Rails
Region	nrt (Tokyo)	Primary deployment region
Release command	bin/rails db:prepare	Runs migrations before starting the app
Health check	GET /up	Rails built-in health endpoint
VM	shared-cpu-1x, 512 MB	Minimal resources for demo purposes
Auto-stop	Enabled	Machines stop when idle to reduce costs

The Dockerfile's entrypoint also runs db:prepare before starting the server. The release command ensures migrations complete before any instances start.

Initial Setup

1. Install the Fly CLI: curl -L https://fly.io/install.sh | sh
2. Sign in: fly auth login
3. Create the app:

   fly apps create rails-preview-demo

4. Create a PostgreSQL cluster:

   fly postgres create --name rails-preview-demo-db --region nrt

5. Attach PostgreSQL to the app:

   fly postgres attach rails-preview-demo-db --app rails-preview-demo

6. Set secrets:

   fly secrets set RAILS_MASTER_KEY=<value from config/master.key> --app rails-preview-demo

7. Deploy:

   fly deploy

GitHub Actions Setup

Two workflows are included:

• fly-deploy.yml — Deploys to production on push to main
• fly-preview.yml — Creates/updates/destroys preview apps for pull requests

Add the following secrets to your GitHub repository (Settings > Secrets and variables > Actions):

Secret	Description
FLY_API_TOKEN	Fly.io org-level token (get via fly tokens create org)
RAILS_MASTER_KEY	Value from config/master.key

PR Preview Environments

Unlike Railway, Fly.io does not have built-in PR preview environments. Instead, the fly-preview.yml workflow uses the superfly/fly-pr-review-apps GitHub Action to:

• Create an isolated Fly app for each pull request
• Attach a new database to the shared PostgreSQL cluster
• Set required secrets (RAILS_MASTER_KEY)
• Run db:prepare via the release command
• Destroy the app when the PR is merged or closed

The preview URL is posted as a comment on the pull request by the action.

Troubleshooting

"app not found" or random app name on first deploy

Fly.io's built-in GitHub integration requires fly.toml to be present on the main branch. Without it, Fly.io's scanner auto-detects the app type and generates a random app name (e.g., app-black-rain-41) that doesn't match any existing app. Ensure fly.toml with the correct app name is merged to main before connecting the repository in the Fly.io dashboard.

"No access token available. Please login with 'flyctl auth login'"

The FLY_API_TOKEN GitHub Secret is missing or empty. Add it in Settings > Secrets and variables > Actions in your GitHub repository.

"Not authorized to deploy this app"

The PR preview workflow creates new Fly apps for each pull request (e.g., pr-7-codenote-net-rails-preview-demo). A deploy token created with fly tokens create deploy is scoped to a single app and cannot create new apps. Use an org-level token instead:

fly tokens create org

Update the FLY_API_TOKEN GitHub Secret with this org-level token.

License

This project is licensed under the MIT License.