Never Fear, Traits are Here

Josh Clayton

I can’t count the number of times I’ve worked on Rails apps where I’m using Factory Bot and want to declare a factory with multiple parents. We’ve all been there, don’t lie.

    FactoryBot.define do
      factory :user do
        name { "Friendly User" }

        factory :admin do
          admin { true }
        end

        factory :male_user do
          gender { "Male" }
          name   { "John Doe" }

          factory :male_admin_user do
            admin { true }
          end

          factory :teenage_male_user do
            date_of_birth { 15.years.ago }

            factory :teenage_admin_male_user do
              admin { true }
            end
          end
        end

        factory :female_user do
          gender { "Female" }
          name   { "Jane Doe" }

          factory :female_admin_user do
            admin { true }
          end

          factory :teenage_female_user do
            date_of_birth { 15.years.ago }

            factory :teenage_admin_female_user do
              admin { true }
            end
          end
        end
      end
    end

Oh, the duplication! This isn’t DRY.

Factory Bot 2.0.41 introduces traits. I’ll cut to the chase:

    FactoryBot.define do
      factory :user do
        name { "Friendly User" }

        trait :admin do
          admin { true }
        end

        trait :male do
          gender { "Male" }
          name   { "John Doe" }
        end

        trait :female do
          gender { "Female" }
          name   { "Jane Doe" }
        end

        trait :teenager do
          date_of_birth { 15.years.ago }
        end

        factory :male_user,                 :traits => [:male]
        factory :female_user,               :traits => [:female]
        factory :teenage_male_user,         :traits => [:teenager, :male]
        factory :teenage_female_user,       :traits => [:teenager, :female]
        factory :male_admin_user,           :traits => [:admin, :male]
        factory :female_admin_user,         :traits => [:admin, :female]
        factory :teenage_admin_male_user,   :traits => [:teenager, :admin, :male]
        factory :teenage_admin_female_user, :traits => [:teenager, :admin, :female]
      end
    end

Mix and match however you please. Bonus: you can assign traits like plain-old attributes.

    FactoryBot.define do
      factory :user do
        name { "Friendly User" }

        trait :admin do
          admin { true }
        end

        trait :male do
          gender { "Male" }
          name   { "John Doe" }
        end

        factory :male_admin_user do
          male
          admin
        end
      end
    end

Many props go out to Thomas Walpole, who helped get a lot of this written; thanks Thomas!

So go ahead, get your trait on; you’ll thank me later.

Project name history can be found here.

  1. FactoryGirl was renamed to FactoryBot in 2017. ↩