thoughtbot--factory_bot/GETTING_STARTED.md

Getting Started

Update Your Gemfile

If you're using Rails:

gem "factory_bot_rails"

If you're not using Rails:

gem "factory_bot"

JRuby users: factory_bot works with JRuby starting with 1.6.7.2 (latest stable, as per July 2012). JRuby has to be used in 1.9 mode, for that, use JRUBY_OPTS environment variable:

export JRUBY_OPTS=--1.9

Once your Gemfile is updated, you'll want to update your bundle.

Configure your test suite

RSpec

If you're using Rails, add the following configuration to spec/support/factory_bot.rb and be sure to require that file in rails_helper.rb:

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

If you're not using Rails:

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

  config.before(:suite) do
    FactoryBot.find_definitions
  end
end

Test::Unit

class Test::Unit::TestCase
  include FactoryBot::Syntax::Methods
end

Cucumber

# env.rb (Rails example location - RAILS_ROOT/features/support/env.rb)
World(FactoryBot::Syntax::Methods)

Spinach

class Spinach::FeatureSteps
  include FactoryBot::Syntax::Methods
end

Minitest

class Minitest::Unit::TestCase
  include FactoryBot::Syntax::Methods
end

Minitest::Spec

class Minitest::Spec
  include FactoryBot::Syntax::Methods
end

minitest-rails

class ActiveSupport::TestCase
  include FactoryBot::Syntax::Methods
end

If you do not include FactoryBot::Syntax::Methods in your test suite, then all factory_bot methods will need to be prefaced with FactoryBot.

Defining factories

Each factory has a name and a set of attributes. The name is used to guess the class of the object by default:

# This will guess the User class
FactoryBot.define do
  factory :user do
    first_name { "John" }
    last_name  { "Doe" }
    admin { false }
  end
end

It is also possible to explicitly specify the class:

# This will use the User class (otherwise Admin would have been guessed)
factory :admin, class: User

If the constant is not available (if you are using a Rails engine that waits to load models, for example), you can also pass a symbol or string, which factory_bot will constantize later, once you start building objects:

# It's OK if Doorkeeper::AccessToken isn't loaded yet
factory :access_token, class: "Doorkeeper::AccessToken"

Because of the block syntax in Ruby, defining attributes as Hashes (for serialized/JSON columns, for example) requires two sets of curly brackets:

factory :program do
  configuration { { auto_resolve: false, auto_define: true } }
end

It is highly recommended that you have one factory for each class that provides the simplest set of attributes necessary to create an instance of that class. If you're creating ActiveRecord objects, that means that you should only provide attributes that are required through validations and that do not have defaults. Other factories can be created through inheritance to cover common scenarios for each class.

Attempting to define multiple factories with the same name will raise an error.

Factories can be defined anywhere, but will be automatically loaded after calling FactoryBot.find_definitions if factories are defined in files at the following locations:

test/factories.rb
spec/factories.rb
test/factories/*.rb
spec/factories/*.rb

Using factories

factory_bot supports several different build strategies: build, create, attributes_for and build_stubbed:

# Returns a User instance that's not saved
user = build(:user)

# Returns a saved User instance
user = create(:user)

# Returns a hash of attributes that can be used to build a User instance
attrs = attributes_for(:user)

# Returns an object with all defined attributes stubbed out
stub = build_stubbed(:user)

# Passing a block to any of the methods above will yield the return object
create(:user) do |user|
  user.posts.create(attributes_for(:post))
end

No matter which strategy is used, it's possible to override the defined attributes by passing a hash:

# Build a User instance and override the first_name property
user = build(:user, first_name: "Joe")
user.first_name
# => "Joe"

Note that objects created with build_stubbed cannot be serialized with Marshal.dump, since factory_bot defines singleton methods on these objects.

Static Attributes

Static attributes (without a block) are no longer available in factory_bot 5. You can read more about the decision to remove them in this blog post.

Aliases

factory_bot allows you to define aliases to existing factories to make them easier to re-use. This could come in handy when, for example, your Post object has an author attribute that actually refers to an instance of a User class. While normally factory_bot can infer the factory name from the association name, in this case it will look for an author factory in vain. So, alias your user factory so it can be used under alias names.

factory :user, aliases: [:author, :commenter] do
  first_name { "John" }
  last_name { "Doe" }
  date_of_birth { 18.years.ago }
end

factory :post do
  author
  # instead of
  # association :author, factory: :user
  title { "How to read a book effectively" }
  body { "There are five steps involved." }
end

factory :comment do
  commenter
  # instead of
  # association :commenter, factory: :user
  body { "Great article!" }
end

Dependent Attributes

Attributes can be based on the values of other attributes using the evaluator that is yielded to dynamic attribute blocks:

factory :user do
  first_name { "Joe" }
  last_name  { "Blow" }
  email { "#{first_name}.#{last_name}@example.com".downcase }
end

create(:user, last_name: "Doe").email
# => "joe.doe@example.com"

Transient Attributes

There may be times where your code can be DRYed up by passing in transient attributes to factories.

factory :user do
  transient do
    rockstar { true }
    upcased { false }
  end

  name { "John Doe#{" - Rockstar" if rockstar}" }
  email { "#{name.downcase}@example.com" }

  after(:create) do |user, evaluator|
    user.name.upcase! if evaluator.upcased
  end
end

create(:user, upcased: true).name
#=> "JOHN DOE - ROCKSTAR"

Transient attributes will be ignored within attributes_for and won't be set on the model, even if the attribute exists or you attempt to override it.

Within factory_bot's dynamic attributes, you can access transient attributes as you would expect. If you need to access the evaluator in a factory_bot callback, you'll need to declare a second block argument (for the evaluator) and access transient attributes from there.

Method Name / Reserved Word Attributes

If your attributes conflict with existing methods or reserved words (all methods in the DefinitionProxy class) you can define them with add_attribute.

factory :dna do
  add_attribute(:sequence) { 'GATTACA' }
end

factory :payment do
  add_attribute(:method) { 'paypal' }
end

Inheritance

You can easily create multiple factories for the same class without repeating common attributes by nesting factories:

factory :post do
  title { "A title" }

  factory :approved_post do
    approved { true }
  end
end

approved_post = create(:approved_post)
approved_post.title    # => "A title"
approved_post.approved # => true

You can also assign the parent explicitly:

factory :post do
  title { "A title" }
end

factory :approved_post, parent: :post do
  approved { true }
end

As mentioned above, it's good practice to define a basic factory for each class with only the attributes required to create it. Then, create more specific factories that inherit from this basic parent. Factory definitions are still code, so keep them DRY.

Associations

It's possible to set up associations within factories. If the factory name is the same as the association name, the factory name can be left out.

factory :post do
  # ...
  author
end

You can also specify a different factory or override attributes:

factory :post do
  # ...
  association :author, factory: :user, last_name: "Writely"
end

In factory_bot 5, associations default to using the same build strategy as their parent object:

FactoryBot.define do
  factory :author

  factory :post do
    author
  end
end

post = build(:post)
post.new_record?        # => true
post.author.new_record? # => true

post = create(:post)
post.new_record?        # => false
post.author.new_record? # => false

This is different than the default behavior for previous versions of factory_bot, where the association strategy would not always match the strategy of the parent object. If you want to continue using the old behavior, you can set the use_parent_strategy configuration option to false.

FactoryBot.use_parent_strategy = false

# Builds and saves a User and a Post
post = create(:post)
post.new_record?        # => false
post.author.new_record? # => false

# Builds and saves a User, and then builds but does not save a Post
post = build(:post)
post.new_record?        # => true
post.author.new_record? # => false

To not save the associated object, specify strategy: :build in the factory:

FactoryBot.use_parent_strategy = false

factory :post do
  # ...
  association :author, factory: :user, strategy: :build
end

# Builds a User, and then builds a Post, but does not save either
post = build(:post)
post.new_record?        # => true
post.author.new_record? # => true

Please note that the strategy: :build option must be passed to an explicit call to association, and cannot be used with implicit associations:

factory :post do
  # ...
  author strategy: :build    # <<< this does *not* work; causes author_id to be nil

Generating data for a has_many relationship is a bit more involved, depending on the amount of flexibility desired, but here's a surefire example of generating associated data.

FactoryBot.define do

  # post factory with a `belongs_to` association for the user
  factory :post do
    title { "Through the Looking Glass" }
    user
  end

  # user factory without associated posts
  factory :user do
    name { "John Doe" }

    # user_with_posts will create post data after the user has been created
    factory :user_with_posts do
      # posts_count is declared as a transient attribute and available in
      # attributes on the factory, as well as the callback via the evaluator
      transient do
        posts_count { 5 }
      end

      # the after(:create) yields two values; the user instance itself and the
      # evaluator, which stores all values from the factory, including transient
      # attributes; `create_list`'s second argument is the number of records
      # to create and we make sure the user is associated properly to the post
      after(:create) do |user, evaluator|
        create_list(:post, evaluator.posts_count, user: user)
      end
    end
  end
end

This allows us to do:

create(:user).posts.length # 0
create(:user_with_posts).posts.length # 5
create(:user_with_posts, posts_count: 15).posts.length # 15

Generating data for a has_and_belongs_to_many relationship is very similar to the above has_many relationship, with a small change, you need to pass an array of objects to the model's pluralized attribute name rather than a single object to the singular version of the attribute name.

Here's an example with two models that are related via has_and_belongs_to_many:

FactoryBot.define do

  # language factory with a `belongs_to` association for the profile
  factory :language do
    title { "Through the Looking Glass" }
    profile
  end

  # profile factory without associated languages
  factory :profile do
    name { "John Doe" }

    # profile_with_languages will create language data after the profile has
    # been created
    factory :profile_with_languages do
      # languages_count is declared as an ignored attribute and available in
      # attributes on the factory, as well as the callback via the evaluator
      transient do
        languages_count { 5 }
      end

      # the after(:create) yields two values; the profile instance itself and
      # the evaluator, which stores all values from the factory, including
      # ignored attributes; `create_list`'s second argument is the number of
      # records to create and we make sure the profile is associated properly
      # to the language
      after(:create) do |profile, evaluator|
        create_list(:language, evaluator.languages_count, profiles: [profile])
      end
    end
  end
end

This allows us to do:

create(:profile).languages.length # 0
create(:profile_with_languages).languages.length # 5
create(:profile_with_languages, languages_count: 15).languages.length # 15

Polymorphic associations can be handled with traits:

FactoryBot.define do
  factory :video
  factory :photo

  factory :comment do
    for_photo # default to the :for_photo trait if none is specified

    trait :for_video do
      association :commentable, factory: :video
    end

    trait :for_photo do
      association :commentable, factory: :photo
    end
  end
end

This allows us to do:

create(:comment)
create(:comment, :for_video)
create(:comment, :for_photo)

Sequences

Unique values in a specific format (for example, e-mail addresses) can be generated using sequences. Sequences are defined by calling sequence in a definition block, and values in a sequence are generated by calling generate:

# Defines a new sequence
FactoryBot.define do
  sequence :email do |n|
    "person#{n}@example.com"
  end
end

generate :email
# => "person1@example.com"

generate :email
# => "person2@example.com"

Sequences can be used in dynamic attributes:

factory :invite do
  invitee { generate(:email) }
end

Or as implicit attributes:

factory :user do
  email # Same as `email { generate(:email) }`
end

Note that defining sequences as implicit attributes will not work if you have a factory with the same name as the sequence.

And it's also possible to define an in-line sequence that is only used in a particular factory:

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

You can also override the initial value:

factory :user do
  sequence(:email, 1000) { |n| "person#{n}@example.com" }
end

Without a block, the value will increment itself, starting at its initial value:

factory :post do
  sequence(:position)
end

Sequences can also have aliases. The sequence aliases share the same counter:

factory :user do
  sequence(:email, 1000, aliases: [:sender, :receiver]) { |n| "person#{n}@example.com" }
end

# will increase value counter for :email which is shared by :sender and :receiver
generate(:sender)

Define aliases and use default value (1) for the counter

factory :user do
  sequence(:email, aliases: [:sender, :receiver]) { |n| "person#{n}@example.com" }
end

Setting the value:

factory :user do
  sequence(:email, 'a', aliases: [:sender, :receiver]) { |n| "person#{n}@example.com" }
end

The value just needs to support the #next method. Here the next value will be 'a', then 'b', etc.

Sequences can also be rewound with FactoryBot.rewind_sequences:

sequence(:email) {|n| "person#{n}@example.com" }

generate(:email) # "person1@example.com"
generate(:email) # "person2@example.com"
generate(:email) # "person3@example.com"

FactoryBot.rewind_sequences

generate(:email) # "person1@example.com"

This rewinds all registered sequences.

Traits

Traits allow you to group attributes together and then apply them to any factory.

factory :user, aliases: [:author]

factory :story do
  title { "My awesome story" }
  author

  trait :published do
    published { true }
  end

  trait :unpublished do
    published { false }
  end

  trait :week_long_publishing do
    start_at { 1.week.ago }
    end_at { Time.now }
  end

  trait :month_long_publishing do
    start_at { 1.month.ago }
    end_at { Time.now }
  end

  factory :week_long_published_story,    traits: [:published, :week_long_publishing]
  factory :month_long_published_story,   traits: [:published, :month_long_publishing]
  factory :week_long_unpublished_story,  traits: [:unpublished, :week_long_publishing]
  factory :month_long_unpublished_story, traits: [:unpublished, :month_long_publishing]
end

Traits can be used as implicit attributes:

factory :week_long_published_story_with_title, parent: :story do
  published
  week_long_publishing
  title { "Publishing that was started at #{start_at}" }
end

Note that defining traits as implicit attributes will not work if you have a factory or sequence with the same name as the trait.

Traits that define the same attributes won't raise AttributeDefinitionErrors; the trait that defines the attribute latest gets precedence.

factory :user do
  name { "Friendly User" }
  login { name }

  trait :male do
    name { "John Doe" }
    gender { "Male" }
    login { "#{name} (M)" }
  end

  trait :female do
    name { "Jane Doe" }
    gender { "Female" }
    login { "#{name} (F)" }
  end

  trait :admin do
    admin { true }
    login { "admin-#{name}" }
  end

  factory :male_admin,   traits: [:male, :admin]   # login will be "admin-John Doe"
  factory :female_admin, traits: [:admin, :female] # login will be "Jane Doe (F)"
end

You can also override individual attributes granted by a trait in subclasses.

factory :user do
  name { "Friendly User" }
  login { name }

  trait :male do
    name { "John Doe" }
    gender { "Male" }
    login { "#{name} (M)" }
  end

  factory :brandon do
    male
    name { "Brandon" }
  end
end

Traits can also be passed in as a list of symbols when you construct an instance from factory_bot.

factory :user do
  name { "Friendly User" }

  trait :male do
    name { "John Doe" }
    gender { "Male" }
  end

  trait :admin do
    admin { true }
  end
end

# creates an admin user with gender "Male" and name "Jon Snow"
create(:user, :admin, :male, name: "Jon Snow")

This ability works with build, build_stubbed, attributes_for, and create.

create_list and build_list methods are supported as well. Just remember to pass the number of instances to create/build as second parameter, as documented in the "Building or Creating Multiple Records" section of this file.

factory :user do
  name { "Friendly User" }

  trait :admin do
    admin { true }
  end
end

# creates 3 admin users with gender "Male" and name "Jon Snow"
create_list(:user, 3, :admin, :male, name: "Jon Snow")

Traits can be used with associations easily too:

factory :user do
  name { "Friendly User" }

  trait :admin do
    admin { true }
  end
end

factory :post do
  association :user, :admin, name: 'John Doe'
end

# creates an admin user with name "John Doe"
create(:post).user

When you're using association names that're different than the factory:

factory :user do
  name { "Friendly User" }

  trait :admin do
    admin { true }
  end
end

factory :post do
  association :author, :admin, factory: :user, name: 'John Doe'
  # or
  association :author, factory: [:user, :admin], name: 'John Doe'
end

# creates an admin user with name "John Doe"
create(:post).author

Traits can be used within other traits to mix in their attributes.

factory :order do
  trait :completed do
    completed_at { 3.days.ago }
  end

  trait :refunded do
    completed
    refunded_at { 1.day.ago }
  end
end

Finally, traits can accept transient attributes.

factory :invoice do
  trait :with_amount do
    transient do
      amount { 1 }
    end

    after(:create) do |invoice, evaluator|
      create :line_item, invoice: invoice, amount: evaluator.amount
    end
  end
end

create :invoice, :with_amount, amount: 2

Callbacks

factory_bot makes available four callbacks for injecting some code:

• after(:build) - called after a factory is built (via FactoryBot.build, FactoryBot.create)
• before(:create) - called before a factory is saved (via FactoryBot.create)
• after(:create) - called after a factory is saved (via FactoryBot.create)
• after(:stub) - called after a factory is stubbed (via FactoryBot.build_stubbed)

Examples:

# Define a factory that calls the generate_hashed_password method after it is built
factory :user do
  after(:build) { |user| generate_hashed_password(user) }
end

Note that you'll have an instance of the user in the block. This can be useful.

You can also define multiple types of callbacks on the same factory:

factory :user do
  after(:build)  { |user| do_something_to(user) }
  after(:create) { |user| do_something_else_to(user) }
end

Factories can also define any number of the same kind of callback. These callbacks will be executed in the order they are specified:

factory :user do
  after(:create) { this_runs_first }
  after(:create) { then_this }
end

Calling create will invoke both after_build and after_create callbacks.

Also, like standard attributes, child factories will inherit (and can also define) callbacks from their parent factory.

Multiple callbacks can be assigned to run a block; this is useful when building various strategies that run the same code (since there are no callbacks that are shared across all strategies).

factory :user do
  callback(:after_stub, :before_create) { do_something }
  after(:stub, :create) { do_something_else }
  before(:create, :custom) { do_a_third_thing }
end

To override callbacks for all factories, define them within the FactoryBot.define block:

FactoryBot.define do
  after(:build) { |object| puts "Built #{object}" }
  after(:create) { |object| AuditLog.create(attrs: object.attributes) }

  factory :user do
    name { "John Doe" }
  end
end

You can also call callbacks that rely on Symbol#to_proc:

# app/models/user.rb
class User < ActiveRecord::Base
  def confirm!
    # confirm the user account
  end
end

# spec/factories.rb
FactoryBot.define do
  factory :user do
    after :create, &:confirm!
  end
end

create(:user) # creates the user and confirms it

Modifying factories

If you're given a set of factories (say, from a gem developer) but want to change them to fit into your application better, you can modify that factory instead of creating a child factory and adding attributes there.

If a gem were to give you a User factory:

FactoryBot.define do
  factory :user do
    full_name { "John Doe" }
    sequence(:username) { |n| "user#{n}" }
    password { "password" }
  end
end

Instead of creating a child factory that added additional attributes:

FactoryBot.define do
  factory :application_user, parent: :user do
    full_name { "Jane Doe" }
    date_of_birth { 21.years.ago }
    gender { "Female" }
    health { 90 }
  end
end

You could modify that factory instead.

FactoryBot.modify do
  factory :user do
    full_name { "Jane Doe" }
    date_of_birth { 21.years.ago }
    gender { "Female" }
    health { 90 }
  end
end

When modifying a factory, you can change any of the attributes you want (aside from callbacks).

FactoryBot.modify must be called outside of a FactoryBot.define block as it operates on factories differently.

A caveat: you can only modify factories (not sequences or traits) and callbacks still compound as they normally would. So, if the factory you're modifying defines an after(:create) callback, you defining an after(:create) won't override it, it'll just get run after the first callback.

Building or Creating Multiple Records

Sometimes, you'll want to create or build multiple instances of a factory at once.

built_users   = build_list(:user, 25)
created_users = create_list(:user, 25)

These methods will build or create a specific amount of factories and return them as an array. To set the attributes for each of the factories, you can pass in a hash as you normally would.

twenty_year_olds = build_list(:user, 25, date_of_birth: 20.years.ago)

build_stubbed_list will give you fully stubbed out instances:

stubbed_users = build_stubbed_list(:user, 25) # array of stubbed users

There's also a set of *_pair methods for creating two records at a time:

built_users   = build_pair(:user) # array of two built users
created_users = create_pair(:user) # array of two created users

If you need multiple attribute hashes, attributes_for_list will generate them:

users_attrs = attributes_for_list(:user, 25) # array of attribute hashes

Linting Factories

factory_bot allows for linting known factories:

FactoryBot.lint

FactoryBot.lint creates each factory and catches any exceptions raised during the creation process. FactoryBot::InvalidFactoryError is raised with a list of factories (and corresponding exceptions) for factories which could not be created.

Recommended usage of FactoryBot.lint is to run this in a task before your test suite is executed. Running it in a before(:suite), will negatively impact the performance of your tests when running single tests.

Example Rake task:

# lib/tasks/factory_bot.rake
namespace :factory_bot do
  desc "Verify that all FactoryBot factories are valid"
  task lint: :environment do
    if Rails.env.test?    
      conn = ActiveRecord::Base.connection
      conn.transaction do
        FactoryBot.lint
        raise ActiveRecord::Rollback
      end
    else
      system("bundle exec rake factory_bot:lint RAILS_ENV='test'")
      fail if $?.exitstatus.nonzero?
    end
  end
end

After calling FactoryBot.lint, you'll likely want to clear out the database, as records will most likely be created. The provided example above uses an sql transaction and rollback to leave the database clean.

You can lint factories selectively by passing only factories you want linted:

factories_to_lint = FactoryBot.factories.reject do |factory|
  factory.name =~ /^old_/
end

FactoryBot.lint factories_to_lint

This would lint all factories that aren't prefixed with old_.

Traits can also be linted. This option verifies that each and every trait of a factory generates a valid object on its own. This is turned on by passing traits: true to the lint method:

FactoryBot.lint traits: true

This can also be combined with other arguments:

FactoryBot.lint factories_to_lint, traits: true

You can also specify the strategy used for linting:

FactoryBot.lint strategy: :build

Verbose linting will include full backtraces for each error, which can be helpful for debugging:

FactoryBot.lint verbose: true

Custom Construction

If you want to use factory_bot to construct an object where some attributes are passed to initialize or if you want to do something other than simply calling new on your build class, you can override the default behavior by defining initialize_with on your factory. Example:

# user.rb
class User
  attr_accessor :name, :email

  def initialize(name)
    @name = name
  end
end

# factories.rb
sequence(:email) { |n| "person#{n}@example.com" }

factory :user do
  name { "Jane Doe" }
  email

  initialize_with { new(name) }
end

build(:user).name # Jane Doe

Although factory_bot is written to work with ActiveRecord out of the box, it can also work with any Ruby class. For maximum compatibility with ActiveRecord, the default initializer builds all instances by calling new on your build class without any arguments. It then calls attribute writer methods to assign all the attribute values. While that works fine for ActiveRecord, it actually doesn't work for almost any other Ruby class.

You can override the initializer in order to:

• Build non-ActiveRecord objects that require arguments to initialize
• Use a method other than new to instantiate the instance
• Do crazy things like decorate the instance after it's built

When using initialize_with, you don't have to declare the class itself when calling new; however, any other class methods you want to call will have to be called on the class explicitly.

For example:

factory :user do
  name { "John Doe" }

  initialize_with { User.build_with_name(name) }
end

You can also access all public attributes within the initialize_with block by calling attributes:

factory :user do
  transient do
    comments_count { 5 }
  end

  name "John Doe"

  initialize_with { new(attributes) }
end

This will build a hash of all attributes to be passed to new. It won't include transient attributes, but everything else defined in the factory will be passed (associations, evaluated sequences, etc.)

You can define initialize_with for all factories by including it in the FactoryBot.define block:

FactoryBot.define do
  initialize_with { new("Awesome first argument") }
end

When using initialize_with, attributes accessed from within the initialize_with block are assigned only in the constructor; this equates to roughly the following code:

FactoryBot.define do
  factory :user do
    initialize_with { new(name) }

    name { 'value' }
  end
end

build(:user)
# runs
User.new('value')

This prevents duplicate assignment; in versions of factory_bot before 4.0, it would run this:

FactoryBot.define do
  factory :user do
    initialize_with { new(name) }

    name { 'value' }
  end
end

build(:user)
# runs
user = User.new('value')
user.name = 'value'

Custom Strategies

There are times where you may want to extend behavior of factory_bot by adding a custom build strategy.

Strategies define two methods: association and result. association receives a FactoryBot::FactoryRunner instance, upon which you can call run, overriding the strategy if you want. The second method, result, receives a FactoryBot::Evaluation instance. It provides a way to trigger callbacks (with notify), object or hash (to get the result instance or a hash based on the attributes defined in the factory), and create, which executes the to_create callback defined on the factory.

To understand how factory_bot uses strategies internally, it's probably easiest to just view the source for each of the four default strategies.

Here's an example of composing a strategy using FactoryBot::Strategy::Create to build a JSON representation of your model.

class JsonStrategy
  def initialize
    @strategy = FactoryBot.strategy_by_name(:create).new
  end

  delegate :association, to: :@strategy

  def result(evaluation)
    @strategy.result(evaluation).to_json
  end
end

For factory_bot to recognize the new strategy, you can register it:

FactoryBot.register_strategy(:json, JsonStrategy)

This allows you to call

FactoryBot.json(:user)

Finally, you can override factory_bot's own strategies if you'd like by registering a new object in place of the strategies.

Custom Callbacks

Custom callbacks can be defined if you're using custom strategies:

class JsonStrategy
  def initialize
    @strategy = FactoryBot.strategy_by_name(:create).new
  end

  delegate :association, to: :@strategy

  def result(evaluation)
    result = @strategy.result(evaluation)
    evaluation.notify(:before_json, result)

    result.to_json.tap do |json|
      evaluation.notify(:after_json, json)
      evaluation.notify(:make_json_awesome, json)
    end
  end
end

FactoryBot.register_strategy(:json, JsonStrategy)

FactoryBot.define do
  factory :user do
    before(:json)                { |user| do_something_to(user) }
    after(:json)                 { |user_json| do_something_to(user_json) }
    callback(:make_json_awesome) { |user_json| do_something_to(user_json) }
  end
end

Custom Methods to Persist Objects

By default, creating a record will call save! on the instance; since this may not always be ideal, you can override that behavior by defining to_create on the factory:

factory :different_orm_model do
  to_create { |instance| instance.persist! }
end

To disable the persistence method altogether on create, you can skip_create for that factory:

factory :user_without_database do
  skip_create
end

To override to_create for all factories, define it within the FactoryBot.define block:

FactoryBot.define do
  to_create { |instance| instance.persist! }


  factory :user do
    name { "John Doe" }
  end
end

ActiveSupport Instrumentation

In order to track what factories are created (and with what build strategy), ActiveSupport::Notifications are included to provide a way to subscribe to factories being run. One example would be to track factories based on a threshold of execution time.

ActiveSupport::Notifications.subscribe("factory_bot.run_factory") do |name, start, finish, id, payload|
  execution_time_in_seconds = finish - start

  if execution_time_in_seconds >= 0.5
    $stderr.puts "Slow factory: #{payload[:name]} using strategy #{payload[:strategy]}"
  end
end

Another example would be tracking all factories and how they're used throughout your test suite. If you're using RSpec, it's as simple as adding a before(:suite) and after(:suite):

factory_bot_results = {}
config.before(:suite) do
  ActiveSupport::Notifications.subscribe("factory_bot.run_factory") do |name, start, finish, id, payload|
    factory_name = payload[:name]
    strategy_name = payload[:strategy]
    factory_bot_results[factory_name] ||= {}
    factory_bot_results[factory_name][strategy_name] ||= 0
    factory_bot_results[factory_name][strategy_name] += 1
  end
end

config.after(:suite) do
  puts factory_bot_results
end

Rails Preloaders and RSpec

When running RSpec with a Rails preloader such as spring or zeus, it's possible to encounter an ActiveRecord::AssociationTypeMismatch error when creating a factory with associations, as below:

FactoryBot.define do
  factory :united_states, class: Location do
    name { 'United States' }
    association :location_group, factory: :north_america
  end

  factory :north_america, class: LocationGroup do
    name { 'North America' }
  end
end

The error occurs during the run of the test suite:

Failure/Error: united_states = create(:united_states)
ActiveRecord::AssociationTypeMismatch:
  LocationGroup(#70251250797320) expected, got LocationGroup(#70251200725840)

The two possible solutions are to either run the suite without the preloader, or to add FactoryBot.reload to the RSpec configuration, like so:

RSpec.configure do |config|
  config.before(:suite) { FactoryBot.reload }
end

Using Without Bundler

If you're not using Bundler, be sure to have the gem installed and call:

require 'factory_bot'

Once required, assuming you have a directory structure of spec/factories or test/factories, all you'll need to do is run:

FactoryBot.find_definitions

If you're using a separate directory structure for your factories, you can change the definition file paths before trying to find definitions:

FactoryBot.definition_file_paths = %w(custom_factories_directory)
FactoryBot.find_definitions

If you don't have a separate directory of factories and would like to define them inline, that's possible as well:

require 'factory_bot'

FactoryBot.define do
  factory :user do
    name { 'John Doe' }
    date_of_birth { 21.years.ago }
  end
end