Add Check Ruby Version For Production App as a Heroku TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
 For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter).
 
-_1719 TILs and counting..._
+_1720 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -502,6 +502,7 @@ If you've learned something here, support my efforts writing daily TILs by
 
 ### Heroku
 
+- [Check Ruby Version For Production App](heroku/check-ruby-version-for-production-app.md)
 - [Connect To A Database By Color](heroku/connect-to-a-database-by-color.md)
 - [Deploy A Review App To A Different Stack](heroku/deploy-a-review-app-to-a-different-stack.md)
 - [Diagnose Problems In A Heroku Postgres Database](heroku/diagnose-problems-in-a-heroku-postgres-database.md)

heroku/check-ruby-version-for-production-app.md

@@ -0,0 +1,32 @@
+# Check Ruby Version For Production App
+
+While deploying a fresh Rails app to Heroku recently, I ran into an issue. The
+`it` block argument wasn't working despite being on Ruby 4.0. Or so I thought.
+
+Running the following command reported the Ruby version of that Heroku server
+instance:
+
+```bash
+❯ heroku run -- ruby --version
+Running ruby --version on ⬢ my-app... up, run.3090
+ruby 3.3.9 (2025-07-24 revision f5c772fc7c) [x86_64-linux]
+```
+
+I was on `3.3.9` which must have been the fallback default at the time.
+
+Though I had set the Ruby version in my `.ruby-version` file, I had neglected to
+specify it in the `Gemfile` as well. Once I added it to the `Gemfile` and
+redeployed, my Heroku server instance was running the expected version of Ruby.
+
+```bash
+❯ heroku run -- ruby --version
+Running ruby --version on ⬢ my-app... up, run.5353
+ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [x86_64-linux]
+```
+
+Note: because [I have set `HEROKU_ORGANIZATION` and
+`HEROKU_APP`](set-default-team-and-app-for-project.md) in my environment
+(`.envrc`) for the local copy of the app, I don't need to specify those when
+running the `heroku run` command above.
+
+See `heroku run --help` for more details.