Rails is a powerful framework. You can write a lot of features in a short period of time. In the process you can easily write code that performs poorly.

At OmbuLabs we like to maintain Ruby on Rails applications. In the process of maintaining them, adding features and fixing bugs, we like to improve the code and its performance (because we are good boy scouts!)

Here are some tips based on our experience.

Prefer where instead of select

When you are performing a lot of calculations, you should load as little as possible into memory. Always prefer a SQL query vs. an object’s method call.

There are already quite a few guides in the wild to help with the upgrade of Rails 3.2 to Rails 4.0.

The official Rails guide for upgrading from Rails 3.2 to 4.0 is very thorough. With the recent release of Rails 5.0, apps currently in production running Rails 3.2 should probably be updated to any stable Rails 4 release as soon as possible.

There is even an e-book about upgrading from Rails 3 to 4, which serves as a useful guide to make this upgrade easier, and also helps understand the advantages & disadvantages of this new (soon to be old) version.

However, if you’re using any non-standard gems, you’re mostly on your own. Some gems stopped being maintained before Rails 4 was released, as was the case with CanCan, a well known authorization library. After many open pull requests were left unmerged, CanCanCan was released. It is a community driven effort to have a semi-official fork of CanCan. It serves as a drop-in replacement for people who want to use CanCan after upgrading to Rails 4.