let can enhance readability when used sparingly (1, 2, or maybe 3 declarations) in any given example group, but that can quickly degrade with overuse.
— RSpec Official Documentation

Motivation

I’ve been using Rspec since 2012 and in all this time I’ve never had a really clear picture of how to best use let. I don’t have this issue with any other aspect of Rspec.

Probably our most common Rubocop violation where I work is MultipleMemoizedHelpers (too many let calls). In previous consulting work, I’ve seen this in many other codebases as well.

What to do about it? On one side, you have Thoughtbot (the creators of FactoryBot) and other prominent members of the community arguing that you should never use let, because of the tangled messes they often see with let overuse in large codebases (eliminating let usage isn’t a practical option for us, given our multiple large legacy Rails applications and many teams). On the other side, you have the well regarded Better Specs site recommending let and of course the Rspec docs themselves, but their examples only cover simple cases. Then in the middle, there’s the quote at the top of this page: a somewhat cryptic comment tucked away in the Rspec code documentation, cautioning to use let sparingly.

But what does it mean to use let sparingly? What is the right strategy for reducing the number of MultipleMemoizedHelpers violations? Why should I care? After a lot of research, focusing primarily (but not solely) on advice from Rspec maintainers that I found in various corners of the internet, I formulated the 5 rules below to answer these questions.

Much of the advice in these rules is really about good habits in general with writing tests, through the lens of using let effectively. let is a tool. It’s up to you how to use it.

ℹ️ Note there’s a Claude Code skill waiting for you at the end of the post, so Claude will know how to use let effectively too.

The problem

File this under small problems that take more time than they should to solve, and I couldn’t find an answer with a web search.

Let’s use a simple example. If you have text like this in your translation file (e.g. en.yml):

users:
  new:
    header: "Let's go!"

And then show it in a view template (e.g. app/views/users/new.html.erb):

<h3><%= t('.header') %></h3>

And then try to match it in an Rspec test, you’ll get an error that it couldn’t be found:

expect(response.body).to include(I18n.t('users.new.header'))

Failure/Error: expected "[...]Let's go![...]" to include "Let's go!"

Hello world! It’s time for my first post in over 4 years.

I recently set up a new Rails 7 project with Rspec and looked online for tips, as one does. I’ve set up many Rails projects before, but not yet with Rails 7, and it’s been a while. The top result in Google for “rails 7 with rspec” is currently Adrian Valenzuela’s Setup RSpec on a fresh Rails 7 project. His post was really helpful for me shaking off the rust. So rather than writing another post that’s 80% the same, I’ll just share a few additional tips. Think of this post as a companion piece to Valenzuela’s.