Create New Rails App with Old Rails Version

Sometimes you need to spin up a rails app using an older version of rails. Running rails new will spin up an app with the latest version (which is what you want most of the time). But if you have older gems of rails installed you can create new rails apps by specifying the version wrapped in underscores.

Say your rails entry in your gem list looks like this: rails (7.2.2.2, 7.2.1, 7.1.5.2, 7.1.2, 7.0.8.7). If you want to create a new rails app using 7.0.8.7, you can run (_ before and after the version number!):

$ rails _7.0.8.7_ new new_app

And boom, you've got a new_app created running rails 7.0.8.7.