My holiday project was to upgrade a Rails project I helped create to Rails 5. The BadgeApp [0] is used to certify open source projects as satisfying the Core Infrastructure Initiative's Best Practices Badge. It is open source, so you can see the diff [1] if you're interested. The upgrade took me several hours, and was greatly assisted by our 99% test coverage. Biggest line count change was from needing to explicitly specify params in tests. The biggest annoyance was that Fastly's gem still doesn't support Rails 5, so I needed to fork it [2].
I too am hugely appreciative of the Rails ecosystem.
[0] https://github.com/linuxfoundation/cii-best-practices-badge/ [1] https://github.com/linuxfoundation/cii-best-practices-badge/... [2] https://github.com/fastly/fastly-rails/pull/67
Here are the app statistics:
+----------------------+--------+--------+---------+---------+-----+-------+
| Name | Lines | LOC | Classes | Methods | M/C | LOC/M |
+----------------------+--------+--------+---------+---------+-----+-------+
| Controllers | 687 | 476 | 8 | 59 | 7 | 6 |
| Helpers | 184 | 134 | 0 | 21 | 0 | 4 |
| Models | 711 | 470 | 5 | 55 | 11 | 6 |
| Mailers | 160 | 115 | 3 | 12 | 4 | 7 |
| Javascripts | 405 | 308 | 0 | 35 | 0 | 6 |
| Libraries | 369 | 281 | 0 | 0 | 0 | 0 |
| Tasks | 369 | 281 | 0 | 0 | 0 | 0 |
| Controller tests | 605 | 513 | 6 | 53 | 8 | 7 |
| Helper tests | 56 | 45 | 2 | 7 | 3 | 4 |
| Model tests | 398 | 309 | 9 | 37 | 4 | 6 |
| Mailer tests | 71 | 54 | 4 | 5 | 1 | 8 |
| Integration tests | 614 | 446 | 10 | 25 | 2 | 15 |
+----------------------+--------+--------+---------+---------+-----+-------+
| Total | 4629 | 3432 | 47 | 309 | 6 | 9 |
+----------------------+--------+--------+---------+---------+-----+-------+