What is the purpose of a `transient do` block in FactoryBot factories?

Question

What is the purpose of transient do in FactoryBot factories?

I\'ve seen a lot of factories that begin with something like below.

factory :c         


        

Answer 1

Transient attributes are essentially variables local to the factory that do not persist into the created object.

I have seen two main uses of transient attributes:

• Controlling/altering the creation of related objects (e.g. accident_count).
• Altering values assigned to other attribute assignments (e.g. unsold).

You could, of course, use these transient attributes for anything else you need to code during the object creation.

factory :car do
  transient do
    accident_count 0
    unsold false
  end

  owner unsold ? 'new inventory' : nil

  after(:create) do |car, evaluator|
    create_list(:police_report, evaluator.accident_count, vehicle: car) 
  end      
end

This lets your test express a concept (similar to a trait), without knowing anything about the implementation:

FactoryBot.create(:car, make: 'Saturn', accident_count: 3)
FactoryBot.create(:car, make: 'Toyota', unsold: true)

IMO, I would stick with traits when they work (e.g. unsold, above). But when you need to pass a non-model value (e.g. accident_count), transient attributes are the way to go.