Did you already find yourself trying to understand a complete rabbit hole code, or do you want to test your brand new bug fix without running a bunch of flows and business rules that may involve third-party endpoint calls?

Let’s try tasting small pieces instead of eating the whole pie.

Why? What’s the benefit?

I already spent countless hours of my day just trying to prove that my code works by running the step-by-step to reproduce the issue, and in the end, I found myself very stressed because most of the time, you’ll miss setting some attributes that will conditionally trigger your fix. Voila, stress level increased.

In my personal experience, the benefits are these:

  • Faster development/comprehensibility
  • Faster QA
  • Faster CI/CD
  • It is more straightforward to track down a flow throughout the codebase

The painful process

I need to create a new user, set their config to this, update this row, update this other thing, and make sure the payload is that… And while you are thinking through all of this, you have to answer some DM’s, monitor logs, etc.

There may be a more straightforward way to test your code that doesn’t require all these painful steps.

Let’s play with some code

Consider this concise ruby class that calculates the BMI number and returns true/false if the user is underweight (you just added this functionality).

class User
  def initialize(weight:, height:)
    @weight = weight
    @height = height

  def bmi
    (@weight / (@height * @height)).round(2)

  def underweight?
    bmi < 18.5

Here are the unit tests:

  • Test if the BMI amount is the expected one for the given weight and height
  • Test if the given weight and height is underweight or not
class TestUser < Test::Unit::TestCase
  def test_bmi
    assert_equal(15.43, User.new(weight: 50, height: 1.80).bmi)

  def test_underweight?
    assert_equal(true, User.new(weight: 50, height: 1.80).underweight?)
    assert_equal(false, User.new(weight: 80, height: 1.80).underweight?)

So, how can I prove that this #underweight? method work as expected? Someone may say: “It’s easy. I know exactly the inputs I need to give to test that method, and I know the output value by calling the #bmi method.” Yes, you’re completely right, It’s already known all the business rules and the flow as well:

user = User.new(weight: 50, height: 1.80)

#> 15.43
user.underweight? # 15.43 is less than 18.5
#> true

Let’s forget about the #bmi for a brief moment so that it doesn’t exist anymore. Now, we are entirely blind about what the calculation is. So, how can you prove the #underweight? method works?

Take It Easy

We don’t need to know the math behind the BMI calculation. We only need to know the conditions that make the #underweight? respond precisely to what we want. If the BMI is 15, the expected output is true since you have this condition: x < 18.5 .

We already know this condition, so we can hard-code the BMI amount and test our method again:

def underweight?
  15 < 18.5

That looks simple, and it is what it is supposed to be, though. It’s the essence of unit testing: mocking values and method’s responses to make the other X method respond to what our automated tests are looking for, and at the end, everything is tested by attacking small pieces per time.

Now we just have proven that #underweight? the method works without having to instance the class, providing the right inputs, learning the calculation behind it, and so on. Get the target condition and hard-code the expected information to see Its desired behavior.

Either 15 or < 18.5 could be a completely different condition that could call a lot of methods in the chain, and you would need to know precisely the inputs, Their formats, and conditions behind the scenes.


I’ve been doing this practice for a long time whenever I need to, and since I got this insight, I try to use this approach in my tickets as much as possible. It’s effortless and helps me to have an general view depending on the case because I can keep iterating the perspective like:

  • Understanding the small pieces
  • Take a step further and understand the small pieces from a different part
  • Comprehend the whole thing without needing to run the entire thing at once

It’s very welcome to take advantage of the unit-testing idea and run that outside of the testing suite in the real world to be your proof of concepts.

Unit testing involves testing small pieces of code to ensure proper functionality overall. Let’s do It then.