Having a clear idea of how much test coverage your Rails application has is really important. Especially if you are planning to upgrade to a newer version of Rails. A good test suite will tell you if your application is working as it did before the upgrade.

At FastRuby.io, we recommend having at least 80% of your application covered before attempting to upgrade. A number lower than that would require you to make a lot more manual testing to ensure that the application is properly working after the upgrade. If your application doesn’t meet that number, we suggest to first spend some time improving the test suite before starting the upgrade.

In this article, I’ll show you how you can measure your test coverage using SimpleCov.

A quick note before starting: SimpleCov doesn’t work in projects with Ruby 1.8.7 or lower. If that’s your case, you can try rcov.

As part of our Roadmap service at FastRuby.io, we have to analyze the test suite of the application we are upgrading to give a proper estimate on how long it will take us to upgrade. We use SimpleCov for this.

Most of our clients use parallelization in their continuous integration tools. SimpleCov generates multiple .resultset.json files for the same codebase. Our goal was to have a single result for the whole application, so in this blog post we are going to show you how we solved that problem.