I’m having a lot of fun with automated code rewrite.

I’d like to create something that I can implement an experiment, change some code and run some test to check if it’s working.

I worked on a simple script to speed up the test suite automatically replaces create with build_stubbed, and it worked. You can check the first part of my novel in my Adventures on automated code replacement.

Now my idea is to expand it in a set of tools that we can use to check if we forgot something or if we can improve something, or remove some code.

So, let me list a few ideas I already worked on.

Replace FactoryGirl.create with build_stubbed

This is a small improvement that fabricates an object but does not make it touches in the database and make specs run faster.

Remove extra options described in RSpec blocks

Lets imagine you’re using timecop-rspec gem and you have a case like this:

it 'travels but check nothing', travel: Time.new(2014, 11, 15) do
  expect(something).to be :ok

But the travel was forgotten there and is not being used. Then we can remove the travel block and run the spec and check if it works.

it 'travels but check nothing' do
  expect(something).to be :ok


If the test passes, the script can make the change official.


My actual script replaced it with all occurrences at one time. Then, it’s a bit weird and is not so powerful because sometimes we have a few cases that can work and others not. Ideally, it should run one change each time and check if that change works.

Then if it works, it can do another change and check if it works.

I also did another very similar script for factory_girl-seeds and in at that time the way I was trying to automate the replacement of FactoryGirl.create to use seed instead. It was a bit bugged because seed method does not support traits and recognize extra attributes as keys for each kind of record needed.

The engine

A tool for run code patch experiments. It should make code changes, create some temporary spec and check if it works. Also, apply to the official code if the spec works.

Criteria for keep the change

The criteria should be related to keep or not keep that change. If the test passes but it’s degrading the performance, or the improvement is insignificant.

Run a command to test the change

Every change should have a way to check if the shift was conclusively positive.

It can be:

One thing I found very weak in my current implementations that I was always running the entire file and I could try to run some specs in cascade. Then I think it should follow some directives:

Divide and conquer

Each experiment can use an isolated environment and without the interference of other checks. In that way, I think we can distribute the labor to multiple workers that can be replacing and reporting the results and even build a third meta-experiment to check if two examples are compatible.


describe User, travel: '2020-02-02' do
  let(:user) { create(user, twitter: 'jonatasdp')}
  it { expect(user).to be_valid }

Lets imagine the experiments in action:

Remove travel

describe User do
  let(:user) { create(user, twitter: 'jonatasdp') }
  it { expect(user).to be_valid }

Replace create with build_stubbed

describe User do
  let(:user) { build_stubbed(user, twitter: 'jonatasdp') }
  it { expect(user).to be_valid }

Remove extra parameters from FactoryGirl create or build_stubbed

describe User do
  let(:user) { build_stubbed(user) }
  it { expect(user).to be_valid }

Learning from mistakes

It can start reporting the success and failures with the current related AST / context to try to identify some patterns to empower intelligence and make it divide and conquer algorithm to avoiding insist in non-effective replacements for some specific cases.

Putting all things together

I think I need a bit more time before start coding. My original idea is to build the following components:




Glue all parts and set up the experiments and know what files it will run.

Navigate in all the files and prepare the work to the divide and conquer with the experiment with the files.

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Hello there, my name is Jônatas Davi Paganini and this is my personal blog.
I'm developer advocate at Timescale and I also have a few open source projects on github.

Check my talks or connect with me via linkedin / twitter / github / instagram / facebook / strava / meetup.