When upgrading an old Rails app, I want to know what will block the upgrade before I start changing versions.

I built rails-preflight to find those blockers by reading the app's files, without booting it.

Before releasing it, I ran it against real Rails upgrades in Mastodon, Discourse and Forem. All 28 gems it flagged as blockers were upgraded, replaced or dropped in the release that moved Rails.

rails-preflight reads a Rails app and tells you what stands between it and the Rails version you want: gems that won't install, APIs your code still calls that Rails removed, the Ruby each step needs, and what it couldn't check.

gem install rails_preflight rails-preflight 7.2 /path/to/your/app

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It writes an HTML report into the app's directory without running the app or its tests, or modifying existing application files.

Mastodon v3.3.0, checked for Rails 7.0.…