PreviewDrop for Rails

Deploy previews for Ruby on Rails apps

Push a branch, get a live preview URL. PreviewDrop builds your Rails container and posts the link to the pull request, usually within a couple of minutes — no CI scripts, no serverless rewrites.

TL;DR

Connect your repo, push. PreviewDrop detects Rails, builds the image and posts a TLS-terminated PR preview URL. Add db:migrate to your Start command to migrate on boot. Puma runs as a persistent process — Action Cable, background jobs, long-polling endpoints all work the same way they do in production.

What the build needs to do

You don't write this. PreviewDrop detects Railsfrom your repo and builds the container for you — a Dockerfile in your repo is not used. The reference below shows the install, build and start steps a working Rails image needs. If detection gets one wrong, set the matching Install, Build or Start command in Project Settings. Your app must listen on $PORT.

Reference (not read by PreviewDrop)
FROM ruby:3.3-slim

RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs && rm -rf /var/lib/apt/lists/*

WORKDIR /app
COPY Gemfile Gemfile.lock ./
RUN bundle install --without development test

COPY . .
RUN SECRET_KEY_BASE=placeholder bundle exec rails assets:precompile RAILS_ENV=production

EXPOSE 3000
CMD sh -c "bundle exec rails db:migrate && bundle exec rails server -b 0.0.0.0 -p ${PORT:-3000} -e production"

Environment variables

Set these in the Variables tab of your PreviewDrop project. They're encrypted at rest and injected into every preview container at start.

Variable	Example value	Note
SECRET_KEY_BASE	$(bundle exec rails secret)	Required in production mode. Generate one with: bundle exec rails secret
RAILS_ENV	production	Set to production so Puma uses the correct config and assets are served.
RAILS_SERVE_STATIC_FILES	enabled	Rails disables static file serving in production by default. This env var re-enables it.
DATABASE_URL	postgres://user:pass@host:5432/db	Point at a shared dev DB or omit to use SQLite (fine for UI-only previews).

Common gotchas

SECRET_KEY_BASE is required in production mode

Rails raises an error at startup if SECRET_KEY_BASE is missing in production. Generate one with bundle exec rails secret and paste it into the PreviewDrop project variables. For previews, any long random string works — don't reuse your production secret.

RAILS_SERVE_STATIC_FILES must be set

By default Rails does not serve static assets in production — it expects nginx to do that. PreviewDrop containers don't have nginx in front. Set RAILS_SERVE_STATIC_FILES=enabled so the Puma process serves the precompiled assets itself.

Asset precompilation needs a placeholder SECRET_KEY_BASE

The reference build above runs assets:precompile at build time with a throw-away SECRET_KEY_BASE=placeholder. Do the same if you set your own Build command — the real secret comes from your project variables at runtime, not baked into the image.

Node.js required for asset pipeline

The reference build above installs nodejs for Sprockets / importmap. If you use Webpacker or esbuild, commit your JS lockfile and make sure your Build command runs your JS build step before assets:precompile.

Why not Vercel or Netlify?

Rails is a long-running server process — not compatible with serverless functions. Action Cable (WebSockets), Sidekiq workers, long-polling endpoints, and multi-second requests don't fit the serverless model that Vercel and Netlify use for backend code. PreviewDrop runs your app in a real container: Puma, Action Cable, background jobs — the full Rails stack as a PR deploy preview, with the same container shape as your production environment.

Preview your Rails app in 5 minutes

Connect a repo, push a branch, get a URL. No credit card, no trial clock.

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