rails-wayback

Preview and compare Rails UI across git branches and commits, directly in the browser, from inside the developer's own Rails process. Toggle it on and off from the terminal so it never gets in the way of normal development.

Idea

When enabled, rails-wayback overlays a small bar at the bottom of every HTML page of your app:

  • Pick a branch and a commit.
  • Hit Travel.
  • The current URL reloads with ?_wayback_ref=<sha>, and the app renders that exact page using the views/layouts/partials of the chosen ref, while keeping the current controller's data and models.

Design goals:

  • Zero configuration. Drop the gem in, run the installer, turn it on.
  • One process only. Uses the running dev app — no extra Rails server.
  • HTML render, not screenshots.
  • Toggleable from the CLI. Stays inert when you don't need it.

Install

Add the gem to the host app's Gemfile, in the development group:

group :development do
  gem "rails-wayback"
end

Then run the installer:

bin/rails generate rails_wayback:install

That adds a conditional mount to config/routes.rb, an initializer at config/initializers/rails_wayback.rb (optional overrides), and a .gitignore entry for tmp/rails_wayback/.

Usage

Turn the UI on when you need it, off when you don't:

bundle exec rails-wayback on      # enable and inject the bar in every page
bundle exec rails-wayback off     # disable it
bundle exec rails-wayback status  # print current state
bundle exec rails-wayback cache   # report ref cache usage and limits
bundle exec rails-wayback prune   # apply configured LRU cache limits
bundle exec rails-wayback clean   # drop the tmp/rails_wayback ref cache
bundle exec rails-wayback doctor  # check dependencies and app readiness

Same commands as rake tasks: rake wayback:on, wayback:off, wayback:status, wayback:cache, wayback:prune, wayback:clean, wayback:doctor.

Run bin/rails wayback:doctor from a Rails application to check Git and tar, the repository root, cache write access, configured view and asset paths, and the active Rails environment. Warnings do not fail the command; missing dependencies or invalid runtime requirements return a non-zero exit status.

Activation lasts for the current Rails server session. Every new server start begins with rails-wayback disabled, while on and off can still be used at any time without restarting Rails.

With the gem enabled, load any page of your app. A dark bar appears at the bottom with:

  • Current ref — dropdown of trusted branches, remote-tracking branches, and tags already stored in the local repository. Its search box filters the loaded refs by name, label, or type.
  • Current commit — dropdown of the recent commits on that ref. Its search box filters the loaded commits by SHA, subject, author, or date.
  • Travel — reloads the current URL using views from that commit.
  • Return to HEAD — appears while traveling; goes back to live views. It uses a server-side reset endpoint, which is also available directly at /rails-wayback/reset if a historical template prevents the page from rendering.

There is no dedicated /rails-wayback page. The gem stays quiet on assets, ActionCable, and every URL under /rails-wayback/*.json (used by the bar to fetch git data).

How "travel" actually works

rails-wayback never runs a second Rails server. Travel is just:

  1. The bar appends ?_wayback_ref=<sha> to the current URL and reloads.
  2. A before_action, auto-included in every ActionController::Base subclass, sees _wayback_ref and materialises that ref under tmp/rails_wayback/refs/<sha>/ (cached per commit).
  3. The same before_action calls prepend_view_path with the sandbox, so Action View resolves templates from the ref before falling back to the current tree.
  4. Standard Rails asset helpers resolve browser-ready files from the ref's public/ directory to immutable, SHA-specific engine URLs. Missing files fall back to the current Rails asset resolver.
  5. Your controller runs as usual, sets its @ variables, and renders. The visual layer (layouts, partials, ERB) comes from the past; the data and business logic come from the present.

If a historical template references a helper or assign that no longer exists, you'll get the same ActionView error you'd get from any missing piece — the app itself never changes. Open /rails-wayback/reset to clear the travel cookies if that error prevents the toolbar from rendering.

The toolbar tracks templates, partials, layouts, and collections. While traveling it labels a response as historical, current fallback, or mixed so you can tell when Rails filled a missing historical file from the current view tree. It also reports historical public assets and current asset fallbacks discovered through standard Rails helpers.

Historical public assets

While traveling, existing calls to asset_path, image_tag, stylesheet_link_tag, javascript_include_tag, and the other standard Rails asset helpers automatically look for browser-ready files in the selected commit's public/ directory. A historical file is served from a URL such as /rails-wayback/refs/<sha>/assets/images/logo.svg; the SHA prevents browser and proxy caches from mixing revisions. Asset responses reauthorize the ref, reject unsafe paths and escaping symlinks, use immutable caching, and keep a cache lease for the complete streamed response.

If a public file is absent, the normal current Rails resolver remains the fallback and the toolbar reports it. Direct hard-coded URLs such as /assets/application.css bypass Rails helpers and are not rewritten. Historical Propshaft/Sprockets compilation, Sass or TypeScript processing, import maps, and JavaScript/CSS bundlers are not reproduced yet; app/assets continues to be extracted for comparison but is not served by this initial static-file strategy.

Configuration (optional)

config/initializers/rails_wayback.rb:

RailsWayback.configure do |config|
  # config.allowed_environments = %w[development test]
  # config.trusted_ref_patterns = ["refs/heads/*"]
  # config.view_paths  = %w[app/views]
  # config.asset_paths = %w[app/assets public]
  # config.max_commits = 50
  # config.max_cached_refs = 25
  # config.max_cache_bytes = 512 * 1024 * 1024
  # config.max_response_bytes = 2 * 1024 * 1024
end

Rails Wayback refuses to activate outside development and test by default, even if its toggle file exists. Adding another environment is an explicit opt-in; historical templates execute inside the host Rails process, so only trusted refs should be used. Travel accepts only full commit SHAs and, by default, the commit must be reachable from a local branch. Narrow trusted_ref_patterns to branches such as refs/heads/main or refs/heads/release/*, or explicitly add locally stored remote-tracking refs and tags:

config.trusted_ref_patterns = [
  "refs/heads/*",
  "refs/remotes/origin/review/*",
  "refs/tags/preview-*"
]

Matching refs appear in the toolbar and can be used for commit discovery. Ref and commit search is performed in the browser over the data already loaded by the toolbar; commit search is therefore limited to max_commits entries for the selected ref and does not run additional Git commands or network requests. Rails Wayback never runs git fetch: remote-tracking refs must already exist locally, symbolic aliases such as origin/HEAD are omitted, and untrusted refs cannot be queried through the commits endpoint. Detached commits remain blocked.

This policy reduces accidental execution but is not a sandbox. Historical ERB and other executable template handlers run with the Rails server's filesystem, database, credentials, environment, and network permissions. Historical JavaScript also executes in the browser under the development application's origin. The toolbar shows a permanent warning and asks for confirmation on the first travel action in each browser session. The response-size limit applies only to toolbar injection; streaming, encoded, file, partial, and larger responses are returned untouched. Toolbar CSS and JavaScript are served from the mounted engine, so CSP policies should allow same-origin styles, scripts, and fetches. Rails-generated CSP nonces are copied to both asset tags automatically. Everything above has a sensible default. Delete the initializer if you don't need to change anything.

Cached refs use least-recently-used access metadata. After materialising a new ref, Rails Wayback automatically removes the oldest refs until the configured count and byte limits are met. A shared cache lease protects each ref for the complete historical render, so automatic or manual prune and clean operations wait for active previews before removing their files. rails-wayback cache reports logical payload size and file count, while rails-wayback prune applies the limits immediately. rails-wayback clean removes all ref data and per-ref lock files. Set either cache limit to nil to disable it.

Compatibility

Rails Wayback requires Ruby 3.3 or newer and Rails 8.0 or newer. CI runs the complete matrix of Ruby 3.3, 3.4, and 4.0 against Rails 8.0 and 8.1. Compatibility here means that the gem's suite passes on that combination; it does not extend the upstream maintenance or security lifetime of Ruby or Rails.

Development

bundle install
bundle exec rspec

Rails dependency sets are managed with Appraisal. Regenerate their Gemfiles after changing Appraisals, then run an individual Rails line with:

bundle exec appraisal generate
bundle exec appraisal rails-8-0 rspec

Release

Releases are cut by pushing an annotated vX.Y.Z git tag. GitHub Actions runs the specs and pushes the gem to RubyGems using Trusted Publishing (OIDC), so there are no API tokens stored on GitHub and MFA on the RubyGems account is never in the way.

One-time setup

  1. On GitHub, go to Settings → Environments → New environment and create an environment named exactly release. (Optionally add branch protection so only main and tags can deploy to it.)
  2. On rubygems.org:
    • For the very first release, go to Profile → Trusted publishers → Pending trusted publishers → Create and fill in:
      • RubyGem name: rails-wayback
      • Repository owner: MrCesar107
      • Repository name: rails-wayback
      • Workflow filename: release.yml
      • Environment: release
    • For every release after 0.1.0, the pending publisher is promoted to a normal trusted publisher automatically — nothing to configure again.
  3. Make sure MFA is enabled on your RubyGems account (the gemspec sets rubygems_mfa_required = true).

Cutting a release

  1. Update lib/rails_wayback/version.rb following SemVer.
  2. Move the pending entries in CHANGELOG.md from [Unreleased] into a new [X.Y.Z] - YYYY-MM-DD section and update the compare/tag links at the bottom of the file.
  3. Commit both files: 
    git commit -am "Release vX.Y.Z"
  4. Tag and push: 
    git tag -a vX.Y.Z -m "Release vX.Y.Z"
git push origin main --follow-tags
  5. Watch the Release workflow on GitHub. It refuses to publish if the tag doesn't match RailsWayback::VERSION, so a typo in the tag will fail loudly before touching RubyGems.

Manual fallback

If the workflow is broken and you need to publish from your laptop:

gem build rails-wayback.gemspec
gem push rails-wayback-X.Y.Z.gem

gem push will ask for your RubyGems credentials and MFA code the first time.

License

MIT