Mind-Bending Factories

People often forget that FactoryBot¹ is just using Ruby to instantiate objects. Most of the time, yes, you’ll use FactoryBot in conjunction with your favorite ORM to instantiate objects for testing Rails apps.

Want a simplistic stub?

FactoryBot.define do
  factory :tiny_stub, class: OpenStruct
end

# >> user = FactoryBot.build(:tiny_stub, admin?: true, name: 'John Doe')
# => #<OpenStruct admin?=true, name="John Doe">
# >> user.admin?
# => true
# >> user.name
# => "John Doe"

Let’s go a bit crazy: what about creating a factory that generates URLs? Writing out valid string URLs is a pain and copy/paste seems both problematic and error-prone. If I want to change the protocol to https:// or switch a subdomain, it gets rougher. Throw in ports and paths and you’re itching for a headache.

So, a factory for generating URLs. First thing’s first: what are the properties we need?

factory :url do
  protocol { 'http://' }
  host { 'www.example.com' }
  port { '80' }
end

That’s a start.

# >> FactoryBot.create(:url)
# NameError: uninitialized constant Url

Ah, forgot that we want this to be an instance of string.

factory :url, class: String do
  protocol { 'http://' }
  host { 'www.example.com' }
  port { '80' }
end

# >> FactoryBot.create(:url)
# NoMethodError: undefined method `protocol=' for "":String

Makes sense; let’s build this string manually.

factory :url, class: String do
  ignore do
    protocol { 'http://' }
    host { 'www.example.com' }
    port { '80' }
  end

  initialize_with { new("#{protocol}#{host}:#{port}") }
end

# >> FactoryBot.create(:url)
# NoMethodError: undefined method `save!' for "http://www.example.com:80":String

Let’s tell the factory to skip creation when we call create:

factory :url, class: String do
  skip_create

  ignore do
    protocol { 'http://' }
    host { 'www.example.com' }
    port { '80' }
  end

  initialize_with { new("#{protocol}#{host}:#{port}") }
end

# >> FactoryBot.create(:url)
# => "http://www.example.com:80"

This is looking better.

What don’t I like about this right now? I’d have to specify protocol manually if I want to make it secure:

# >> FactoryBot.create(:url, protocol: 'https://', port: nil)
# => "https://www.example.com:"

Yuck; it’s all sorts of broken. Let’s use a trait to make this more managable.

factory :url, class: String do
  skip_create

  ignore do
    protocol { 'http://' }
    host { 'www.example.com' }
    port { '80' }
  end

  trait :secure do
    protocol { 'https://' }
    port { nil }
  end

  initialize_with { new("#{protocol}#{[host, port].compact.join(':')}") }
end

# >> FactoryBot.create(:url, :secure)
# => "https://www.example.com"

Up next: subdomain and domain name!

factory :url, class: String do
  skip_create

  ignore do
    protocol { 'http://' }
    subdomain { 'www' }
    domain_name { 'example.com' }

    host { [subdomain, domain_name].compact.join('.') }
    port { '80' }
  end

  trait :secure do
    protocol { 'https://' }
    port { nil }
  end

  initialize_with { new("#{protocol}#{[host, port].compact.join(':')}") }
end

# >> FactoryBot.create(:url, subdomain: 'blog')
# => "http://blog.example.com:80"

Finally, path:

factory :url, class: String do
  skip_create

  ignore do
    protocol { 'http://' }
    subdomain { 'www' }
    domain_name { 'example.com' }

    host { [subdomain, domain_name].compact.join('.') }
    port { '80' }

    path { '/' }
  end

  trait :secure do
    protocol { 'https://' }
    port { nil }
  end

  initialize_with { new("#{protocol}#{[host, port].compact.join(':')}#{path}") }
end

# >> FactoryBot.create(:url, path: '/about/us')
# => "http://www.example.com:80/about/us"

So, what’s this give us?

# >> FactoryBot.create(:url)
# => "http://www.example.com:80/"
# >> FactoryBot.create(:url, :secure)
# => "https://www.example.com/"
# >> FactoryBot.create(:url, :secure, subdomain: 'blog', path:
# >> '/12345/great-post-title')
# => "https://blog.example.com/12345/great-post-title"
# >> FactoryBot.create(:url, domain_name: 'example.co.uk', port: '1234')
# => "http://www.example.co.uk:1234/"

For more fun(ctionality), we can add child factories:

factory :url, class: String do
  # ...

  factory :twitter do
    secure
    subdomain { nil }
    domain_name { 'twitter.com' }
  end

  factory :facebook do
    secure
    subdomain { nil }
    domain_name { 'facebook.com' }
  end

  factory :google_analytics do
    domain_name { 'google.com' }
    port { nil }
    path { '/analytics' }
  end
end

# >> FactoryBot.create(:twitter)
# => "https://twitter.com/"
# >> FactoryBot.create(:facebook)
# => "https://facebook.com/"
# >> FactoryBot.create(:google_analytics)
# => "http://www.google.com/analytics"

FactoryBot is really powerful. With the ability to specify the factory’s class, traits, initialize_with, skip_create, and dynamic attributes, we can build a flexible factory that can be used whenever you need URLs - all without having to build strings by hand.

What custom factories have you built?

Project name history can be found here.

Looking for FactoryGirl? The library was renamed in 2017. ↩

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