Two decisions on the top of my head:
- I wanted to make an index page where a user could make edits to the items being displayed and make regular show/edit pages for the item so if a user was on that page they could edit it. This is actually really useful: a user can bookmark the page for a specific item, or open it in a new tab, or in general do things browsers let you do but apps struggle with. Making two different edit components would be stupid, and I thought this would be one of those things that I could do better in React than in Rails. After looking around a bit I quickly found that if I used standard REST-y routes and wrapped the key parts of the view with turbo frames I could get exactly what I wanted, and it worked out seamlessly. In general, I've found that Rails' heavy emphasis on REST was a good architectural choice, and every time I disagreed with it and went another way I ended up regretting that choice and reverting to REST.
- In the Java world, you typically have thin models + a service layer which has the business logic. This is apart from other layers such as Repo/DAO etc, which I was already replacing with ActiveRecord, but I was initially resistant to putting all of my business logic in the models, especially if it involved logic across them. But it also hurt discoverability. I was working on a project I was expecting other people to work on with me, and I wanted to make it easy for them to figure out what they could do with a model object. The solution to this came from DHH himself. I've lost a link for this, but he said that if you make service objects, you should add a method on the model that acts as a way to reach the service. This keeps the model "fat" while also separating out logic into simple, unit-testable classes.
One of the smaller things I appreciate is keeping all of the routing details in rails routes. I know other frameworks like Django also do this, but I really didn't like this about most Java frameworks and microframeworks in other languages.
In general, if you're interested in seeing how to architect Rails apps, I say study how 37Signals does it. Playing around with Basecamp convinced me of the practicality of the Rails way and taught me a lot of interesting and useful patterns.