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

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  endend

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.

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.