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 :car do
  owner nil
  other_attribute nil
end
...

I've found some information on this blog:
Using FactoryGirl to easily create complex data sets in Rails

But I still don't fully understand how and why to do this. My experience with FactoryBot is minimal.

Could anyone with some experience using FactoryBot share some insight?

Answer 1

transient attributes allow you to pass in data that isn’t an attribute on the model.

Say you have a model called car with the following attributes:

• name
• purchase_price
• model

You want to capitalize the name of the car when you create the car model in the factory. What we can do is:

factory :car do
  transient do
    # capitalize is not an attribute of the car
    capitalize  false
  end

  name           { "Jacky" }
  purchase_price { 1000 }
  model          { "Honda" }

  after(:create) do |car, evaluator|
    car.name.upcase! if evaluator.capitalize
  end
end

Hence, whenever you create the car factory and you want to capitalize the name. You can do

car = FactoryGirl.create(:car, capitalize: true)
car.name
# => "JACKY"

Hope it helps.

Answer 2

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.