Does Rails rake db:migrate needs a javascript runtime?

Question

In my first rails project (using mysql) I tried to execute the rake db:create command, but it outputted the following error:

Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes.

So I included gem 'therubyracer', require: "v8" on my Gemfile and executed a bundle install. The problem is solved, but it left some doubts.

Could somebody give me an explanation about why a javascript runtime is needed for this kind of task? I'm new to Ruby and RoR so this seems quite strange to me, why do RoR use javascript for creating a database?

Answer 1

You need a javascript runtime because now Rails uses an asset pipeline. Rails doesn't need it to run your migrations but when you run rake db:migrate, the rake task loads your rails application that needs a javascript runtime to start. Practically, there is no relation between the migrations and the javascript runtime but your app won't load without it.