[jacobsenscott]

I've upgraded our moderately sized app from 3 to 4 to 5 to 6. Haven't gotten to 7 yet. This is widely accepted to be one of the most painful parts of rails, so I'm surprised you haven't had a problem. Maybe you've only upgraded small apps, or maybe only upgrade very recent versions (they are getting better). There are whole consultancies dedicated just to this task. Github and Shopify post about dedicating whole teams and tools just to the task.

Even with very high test coverage on a large app it is a huge and high risk effort to upgrade - there are so many deprecations and unnecessary behavior changes. The `belongs_to` now validates presence was especially egregious and arbitrary. Of course you can turn it off with a variable - but now we have a rails app that is a mix if defaults from 4 different versions of rails.