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ROM-Factory - Your testing buddy for Hanami apps

Episode #31

by Sebastian Wilgosz

Picture of the author

I've recorded several episodes about ROM, Hanami, and ruby, but none of them touched the testing part from the practical point of view.

In this episode, I'll talk exactly about that. Or, to be more precise, how to quickly instantiate and persist objects for testing purposes, using ROM-factory.

The problem in testing

I have this little project, where I can publish my articles using my interactor class. I have three tests written for it, and to test the interactor well, I need to have some articles persisted in the database.

I have two different articles, one published, and in the other example, unpublished.

let(:article) { repo.create(published_on: Date.parse('2022-09-09')) }

let(:article) { repo.create(published_on: nil) }

The problem with this is, that I have a foreign key set on author's id, and a not null constraint, which in this case, will be violated.

     Failure/Error: let(:article) { repo.create(published_on: Date.parse('2022-09-09')) }

       PG::NotNullViolation: ERROR:  null value in column "author_id" violates not-null constraint
       DETAIL:  Failing row contains (1, null, null, null, null, 2022-09-09, 2022-10-01 00:26:42.124334, 2022-10-01 00:26:42.124334).

Creating objects directly in test examples is problematic because I need to know exactly, which attributes are required, and what relations need to be created.

More importantly, whenever some of it changes, I'd need to run through all my tests and update all the object definitions accordingly.

Wouldn't it be cool, if I'd have this logic extracted to a single place, and never worry if I create a valid object or not?

let(:article) { Factory[:published_article] }

let(:article) { Factory[:draft_article] }

This is where the concept of factories had been born.

FactoryBot for rails applications

If you have a Rails background, I'm sure you've heard about FactoryBot, the popular gem used to quickly build instances of your models, so testing can be fun.

I totally recommend it for rails projects, as I used it a lot and like it very much. However, while they claim to not be rails-specific, I find it very troublesome to use it with non-ActiveRecord projects.

[!QUESTION] Do you know such? If you've met projects using FactoryBot out of the Rails-specific gems or apps, please let me know, I'll gladly take a closer look at this.

The idea behind factories is super cool, and I guess anyone who had a chance to work with FactoryBot or a similar library, would like to see something like this in Hanami or ROM too.

Good news! ROM has a dedicated gem that we can use exactly for this purpose.

ROM-Factory for Hanami apps

In Hanami applications, we use ROM-RB for the persistence layer, and ROM comes with a small neat gem to support those of us who test their code on a daily basis.

It's a ROM-Factory gem, and despite some minor syntax differences, I'm sure you'll find it very similar to whatever you got used so far.

The difference is that ROM-Factory works with ROM relations out of the box, and there is minimal to no configuration required.


To install the gem, you'll need a rom-factory gem added to the Gemfile and run bundle to install it.

# Gemfile

group :development, :test do
  # ..
  gem "rom-factory"

It has a faker gem listed as a dependency, so you can use it out of the box to generate the random data for your fields.


To configure the ROM-Factory in your project - which in my case will be a Hanami application - you just need to add the factory configuration file to your spec support folder and point to the rom container instance.

I'm going to require the faker here, so I don't need to care if it's loaded in the specific factories who use it.

Finally, I need to load the factory definition files, which I'll store in the spec/support/factories folder.

# spec/support/factory.rb

require_relative "rom-factory"
require "faker"

Factory = ROM::Factory.configure do |config|
  config.rom =

Dir[File.dirname(__FILE__) + '/factories/*.rb'].each { |file| require file }

Then load this configuration in the spec_helper file:

# spec/spec_helper.rb

require_relative "support/factory"

Our gem is fully configured now to be used within our test suit! Now we can define some factories for our tests.

Defining factories

First, let me create an article factory. For my test, I won't care about any of the attributes I don't check at the given moment. For now, I'm only interested in creating a draft article, and a published one.

To solve the problem with not-null constraint violation, I'm going to add the association pointing to the author. With this, whenever I create an article using this factory, the author will also be instantiated with the use of the author factory which I'll define right now.

# spec/support/factories/article.rb

Factory.define(:article) do |f|

Factory.define(published_article: :article) do |f|
  f.published_on Date.today

In a new factory file, I'm adding a simple object, filling-in first_name, last_name, and a nickname. The nickname is a unique field, so I'll use the sequence feature to set different values each time the factory is used.

Factory.define(:author) do |f|
  f.first_name Faker::Name.first_name
  f.last_name Faker::Name.last_name
  f.nickname { Faker::Internet.unique.username }

With this, I can now don't care about the exact object initialization logic in my tests, but just use my factory, and hide all that inside!

context 'when given article is already published' do
  let(:article) { Factory[:published_article] }
  # ...

context 'when given article is not published' do
  let(:article) { Factory[:article] }

I know it's not much for this little example, but for more complex applications persisting objects in tests can be extremely annoying.

Why to bother if you don't need to?


ROM-Factory is a nice gem created to make ROM-based applications more testable, and I need to admit, they succeeded with it.

I am a huge fan of testing my code, so I love having this in place. as I got used to the factory_bot before. I hope you'll enjoy it too!

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