factory-bot-data

Overview

FactoryBot (formerly factory_girl) is the canonical Ruby fixture- factory library - it builds object graphs with referential integrity that raw faker-data cannot (factory_bot-readme).

The split:

• Faker generates field values (Faker::Name.name).
• FactoryBot orchestrates object creation (create(:user, :admin, posts_count: 3)) - including sequence-based unique IDs, associations between models, and trait composition.

When to use

• The project is Ruby (Rails or pure Ruby).
• Tests need structured fixtures: a User with three Posts and a Profile, all linked correctly.
• The test suite uses RSpec or Minitest - both are first-class.
• The team wants "intent-revealing" fixture names (create(:admin)) rather than per-field overrides.

How to use

1. Add factory_bot_rails (Rails) or factory_bot to the test group and require it.
2. Define one base factory per model in spec/factories/, one attribute block each.
3. Add trait blocks for variants (:admin, :with_posts) instead of separate variant factories.
4. Wire Faker into attribute blocks for randomized values; use .unique where uniqueness matters.
5. Pick a build strategy per test: build_stubbed for unit, build for no-DB, create for persistence.
6. Include FactoryBot::Syntax::Methods in the RSpec / Minitest config to call create(:user) directly.
7. Call the factory in the test and assert on the returned object.

Install

gem install factory_bot

For Rails projects, prefer the Rails-specific gem with auto-loading:

# Gemfile
group :test do
  gem 'factory_bot_rails'   # adds Rails-specific helpers and auto-loads spec/factories/
end

(Per factory_bot-readme.)

Authoring

Basic factory

# spec/factories/users.rb
FactoryBot.define do
  factory :user do
    name  { "John Doe" }
    email { "john@example.com" }
  end
end

(Per factory_bot-readme.)

Each attribute is a block - the block is evaluated lazily at object creation, so Faker::Name.name re-runs per create call.

Sequences

For unique values across factory invocations:

sequence :email do |n|
  "user#{n}@example.com"
end

factory :user do
  email   # uses the sequence
end

(Per factory_bot-readme.)

Associations

factory :post do
  title  { "Hello" }
  body   { "World" }
  user   # implicit reference to a user factory
end

# OR explicit association
factory :post do
  user   # short form
  # association :user, factory: :admin   # explicit form
end

# Has-many: build N posts from a user factory
factory :user do
  name { "John" }
  after(:create) do |user|
    create_list(:post, 3, user: user)
  end
end

Traits

Traits compose mix-ins onto a base factory:

factory :user do
  name  { "John Doe" }
  email { "john@example.com" }

  trait :admin do
    role  { "admin" }
  end

  trait :with_posts do
    after(:create) do |user|
      create_list(:post, 3, user: user)
    end
  end
end

# Apply one or more traits at create time
FactoryBot.create(:user, :admin, :with_posts)

(Per factory_bot-readme.)

Traits are the canonical way to keep factory bodies DRY - instead of ten variant factories (:admin_user, :premium_user, :admin_premium_user, …), one base factory plus N traits composes to all variants.

Build strategies

FactoryBot supports three: build (unsaved), create (persisted with associations), and build_stubbed (fake-persisted, never hits the DB). Speed hierarchy: build_stubbed >> build >> create - use the weakest one that still tests what you need. Full table and examples: references/strategies-and-anti-patterns.md.

Integrating with Faker

Plug Faker into the factory body for randomized values:

require 'faker'

FactoryBot.define do
  factory :user do
    name  { Faker::Name.name }
    email { Faker::Internet.unique.email }   # `.unique` enforces uniqueness across factory invocations
    age   { Faker::Number.between(from: 18, to: 80) }
  end
end

Faker::Internet.unique (and other unique helpers) tracks generated values per session and raises if exhausted. Use this instead of a sequence when you want both randomness and uniqueness.

Test framework integration

RSpec

# spec/spec_helper.rb (or rails_helper.rb for Rails)
RSpec.configure do |config|
  config.include FactoryBot::Syntax::Methods   # enables `create(:user)` instead of `FactoryBot.create(:user)`
end

# spec/users_spec.rb
RSpec.describe User do
  it 'has a name' do
    user = create(:user)
    expect(user.name).to be_present
  end
end

Minitest

# test/test_helper.rb
class ActiveSupport::TestCase
  include FactoryBot::Syntax::Methods
end

Per factory_bot-readme; both syntaxes mirror the same DSL - only the test-framework hookup differs.

Worked example

A test needs an admin user that owns three posts. Define the factory with two traits and Faker-backed values:

# spec/factories/users.rb
FactoryBot.define do
  factory :user do
    name  { Faker::Name.name }
    email { Faker::Internet.unique.email }

    trait :admin do
      role { "admin" }
    end

    trait :with_posts do
      after(:create) { |user| create_list(:post, 3, user: user) }
    end
  end
end

Compose both traits in one intent-revealing call:

RSpec.describe User do
  it 'admin owns three posts' do
    user = create(:user, :admin, :with_posts)
    expect(user.role).to eq('admin')
    expect(user.posts.count).to eq(3)
  end
end

create persists the user and, via the :with_posts after(:create) hook, its three posts - one line instead of six lines of manual setup.

Anti-patterns

Common FactoryBot pitfalls - create for read-only tests, mega base factories, per-test-class definitions, sequence where values should be random, bulk create_list, and always-create associations - and their fixes are catalogued in references/strategies-and-anti-patterns.md.

Limitations

• Ruby-only. For other languages: factory_boy (Python), fishery (TS), Bogus (.NET), MockK (Kotlin).
• Auto-load only with factory_bot_rails. Pure Ruby projects must require factory files manually.
• No native support for has_many through:. You can express it with after(:create) blocks, but the syntax is hand-rolled.
• Sequence resets on test reload. A long-running suite generates user1@example.com ... user5000@example.com. If asserting on the n-th sequence value, your test breaks when run in isolation.

References

• factory_bot-readme - canonical: install, factory definition, traits, associations, sequences, build / create / build_stubbed strategies.
• faker-data - Ruby Faker (the value engine for FactoryBot fields).
• synthetic-data-tool-selector - dispatcher selecting the right factory library per language.
• seed-data-curator - downstream workflow consuming FactoryBot for E2E suite seeds.