A reader of mine recently asked me how I set up Factory Bot for a new Rails project.

There are four steps I go through to set up Factory Bot.

1. Install the factory_bot_rails gem
2. Set up one or more factory definitions
3. Install Faker
4. Add the Factory Bot syntax methods to my rails_helper.rb file

Following are the details for each step.

Install the factory_bot_rails gem

The first thing I do is to include the factory_bot_rails gem (not the factory_bot gem) in my Gemfile. I include it under the :development, :test group.

Here’s a sample Gemfile from a project with only the default gems plus a few that I added for testing.

Remember that after you add a gem to your Gemfile you’ll need to run bundle install in order to actually install the gem.

source 'https://rubygems.org'
git_source(:github) { |repo| "https://github.com/#{repo}.git" }

ruby '2.7.0'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 6.0.2', '>= 6.0.2.2'
# Use postgresql as the database for Active Record
gem 'pg', '>= 0.18', '< 2.0'
# Use Puma as the app server
gem 'puma', '~> 4.1'
# Use SCSS for stylesheets
gem 'sass-rails', '>= 6'
# Transpile app-like JavaScript. Read more: https://github.com/rails/webpacker
gem 'webpacker', '~> 4.0'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder

gem 'devise'

# Reduces boot times through caching; required in config/boot.rb
gem 'bootsnap', '>= 1.4.2', require: false

group :development, :test do
  gem 'pry'
  gem 'rspec-rails'
  gem 'capybara'
  gem 'webdrivers'
  gem 'factory_bot_rails'
end

group :development do
  # Access an interactive console on exception pages or by calling 'console' anywhere in the code.
  gem 'web-console', '>= 3.3.0'
  gem 'listen', '>= 3.0.5', '< 3.2'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]

Set up one or more factory definitions

Factory definitions are kind of the “templates” that are used for generating new objects.

For example, I have a user object that needs an email and a password, then I would create a factory definition saying “hey, make me a user with an email and password”. The actual code might look like this:

FactoryBot.define do
  factory :user do
    email { 'test@example.com' }
    password { 'password1' }
  end
end

Factory Bot is smart enough to know that when I say factory :user do, I’m talking about an Active Record class called User.

There’s a problem with this way of defining my User factory though. If I have a unique constraint on the users.email column in the database (for example), then I won’t ever be able to generate more than one User object. The first user’s email address will be test@example.com (no problem so far) but then when I go to create a second user, its email address will also be test@example.com, and if I have a unique constraint on users.email, the creation of this second record will not be allowed.

We need a way of making it so the factories’ values can be unique. One way, which I’ve done before, is to append a random number to the end of the email address, e.g. "test#{SecureRandom.hex}@example.com". There’s a different way to do it, though, that I find nicer. That way is to use another gem called Faker.

Install Faker

Just like I showed with factory_bot_rails above, the Faker gem can be added by putting it into the :development, :test group of the Gemfile.

Then we can change our User factory definition as follows.

FactoryBot.define do
  factory :user do
    email { Faker::Internet.email }
    password { Faker::Internet.password }
  end
end

This will give us random values like eldora@jones.net and lazaromertz@ko.name.

Add the Factory Bot syntax methods to my rails_helper.rb file

The syntax for actually using a Factory Bot factory in a test is as follows:

FactoryBot.create(:user)

There’s nothing wrong with this, but I find that these FactoryBot are so numerous in my test files that their presence feels a little noisy.

There’s a way to make it so that instead we can just write this:

create(:user)

The way to do that is to add a bit of code to spec/rails_helper.rb.

RSpec.configure do |config|
  config.include FactoryBot::Syntax::Methods
end

(You don’t actually add the RSpec.configure do |config| to the spec/rails_helper.rb file. It’s already there. I’m just including it here to show that that’s the block inside of which the config.include FactoryBot::Syntax::Methods line goes.)

What to do next

If you’re curious how to put Factory Bot together with the other testing tools to write some complete Rails tests, I might suggest my Rails testing “hello world” tutorial using RSpec and Capybara.