Can FactoryBot generate factories after your models have been created?

First thing, look at the source project to find out how it was implemented:

https://github.com/thoughtbot/factory_bot_rails/blob/master/lib/generators/factory_bot/model/model_generator.rb

After that, try to guess how it works:

rails g factory_bot:model Car name speed:integer

The result is:

create  test/factories/cars.rb

And the content:

# Read about factories at https://github.com/thoughtbot/factory_girl

FactoryBot.define do
   factory :car do
     name "MyString"
     speed 1
   end
end

Remember, when you use rails g, you can always undo it, with rails d

rails d factory_bot:model Car name speed:integer

Note: FactoryBot was previously named FactoryGirl