FactoryBot: build vs. create, Overrides & Traits

FactoryBot (formerly Factory Girl) generates test data via a factory pattern, replacing Rails' built-in static YAML fixtures with a flexible, code-based approach supporting dynamic values and per-test overrides.

Defining a Factory

FactoryBot.define do
  factory :user do
    name { "Ada Lovelace" }
    sequence(:email) { |n| "user#{n}@example.com" }

    trait :admin do
      role { "admin" }
    end
  end
end

# sequence() gives each generated user a unique email,
# avoiding uniqueness-constraint failures across tests

build vs. create

user = FactoryBot.build(:user)   # in-memory only, not saved -- faster
user = FactoryBot.create(:user)  # saved to the database

# Prefer build() whenever the test doesn't actually need
# the record persisted -- skips an unnecessary DB write

Overrides & Traits

FactoryBot.create(:user, name: "Bob")     # override a specific attribute
FactoryBot.create(:user, :admin)          # apply the :admin trait
FactoryBot.create_list(:user, 5)          # create 5 independent users

Why Not Just `Model.new(...)` Everywhere?

When a model gains a new required attribute, a shared factory only needs updating once. Tests manually instantiating objects everywhere would each need individual updates -- a real maintenance burden a shared factory avoids.