PreviewDrop for Rails

A live Rails preview for every branch

Rails apps run end-to-end on PreviewDrop: migrations, Active Job workers, Action Cable websockets, Turbo streams. No serverless rewrite, no cold starts, no Edge Runtime compatibility matrix.

TL;DR

Add a Dockerfile, set RAILS_MASTER_KEY and a DB URL, push. Works with Rails 7.x and 8.x. PG, MySQL, SQLite all supported. Each branch gets its own subdomain — same URL across pushes.

Dockerfile

Drop this at the root of your repo. PreviewDrop picks it up on the next push and builds it into a container behind a TLS-terminated subdomain.

Dockerfile
FROM ruby:3.3-slim
RUN apt-get update -qq && apt-get install -y --no-install-recommends \
    build-essential libpq-dev libvips nodejs \
    && rm -rf /var/lib/apt/lists/*
WORKDIR /app
COPY Gemfile Gemfile.lock ./
RUN bundle install --jobs 4 --retry 3
COPY . .
RUN SECRET_KEY_BASE=dummy bundle exec rake assets:precompile
EXPOSE 3000
CMD ["sh", "-c", "bundle exec rails db:migrate && \
    bundle exec rails s -b 0.0.0.0 -p $PORT"]

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
RAILS_ENV	production	Use production for previews — closer to real behavior than development.
RAILS_MASTER_KEY	<contents of config/master.key>	Required so Rails can decrypt credentials.yml.enc. Never commit the key itself.
DATABASE_URL	postgres://user:pass@dev-db.internal/myapp_preview	Point at a shared preview DB, or pair with Neon for per-branch isolation.
SECRET_KEY_BASE	<rails secret>	Generate with rails secret. Different from RAILS_MASTER_KEY.
RAILS_SERVE_STATIC_FILES	true	Lets Rails serve /assets without nginx in front.
RAILS_LOG_TO_STDOUT	true	Streams logs to PreviewDrop’s runtime logs tab.

Common gotchas

Assets:precompile needs a SECRET_KEY_BASE

Rails 7+ errors on bundle exec rake assets:precompile if SECRET_KEY_BASEis unset. The Dockerfile above sets a dummy value at build time (it's overridden at runtime by the real value from your Variables tab).

force_ssl and redirect loops

PreviewDrop terminates TLS at the edge and proxies HTTP to your container. If your Rails app has config.force_ssl = true, set config.ssl_options = { redirect: false } or trust the X-Forwarded-Protoheader, otherwise you'll hit a redirect loop.

Active Job workers

For Sidekiq or GoodJob workers, create a second PreviewDrop project on the same repo with a worker-specific Dockerfile (CMD ["bundle", "exec", "sidekiq"]). Both projects share env vars and coordinate via Redis. PreviewDrop won't put the worker behind a public URL — set the Exposed port to a non-HTTP value or leave it unset.

ActionMailer in previews

Don't send real email from previews. Set config.action_mailer.delivery_method = :test or use Mailcatcher / letter_opener_web. Previews are ephemeral — a reviewer opening a preview can accidentally trigger welcome emails to real users if you leave prod SMTP config wired up.

Why not Vercel or Netlify?

Vercel doesn't run Rails. You can try rails-on-lambda-style workarounds, but you lose Sidekiq, ActionCable, long-running uploads, and half your middleware. Rails is a fundamentally stateful framework; PreviewDrop runs it the way it expects — as a persistent process in a container.

More Rails patterns

Frameworks & Docker for the full guide;Environment variables for per-branch DB patterns.

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