Add Get Help With A Rails App Update as a Rails TIL

rails/get-help-with-a-rails-app-update.md

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+# Get Help With A Rails App Update
+
+Rails version upgrades can be pretty involved. The ecosystem and the framework
+are under constant evolution. While each patch version will stay reliably
+stable, as soon as you go to do a minor or major upgrade (which you should stay
+on top of), you'll have lots to consider.
+
+Rails helps with this via the `app:update` rake task.
+
+When run, it will prompt you with a series of files that it wants to change.
+For each one you'll have some options.
+
+```bash
+$ rails app:update
+
+Overwrite my-app/config/boot.rb? (enter "h" for help) [Ynaqdhm]  h
+        Y - yes, overwrite
+        n - no, do not overwrite
+        a - all, overwrite this and all others
+        q - quit, abort
+        d - diff, show the differences between the old and the new
+        h - help, show this help
+        m - merge, run merge tool
+```
+
+Start by using `d` to see a diff of the changes. If it's small and doesn't
+overwrite important, existing settings, then you can use `y` to accept them.
+For a lot of these files the changes will be too aggressive. So side-by-side
+with the diff, update the file manually. Then use `n` to go to the next update.