While I’m still not entirely sold on FactoryGirl, I often see it being used in a particularly lazy way. Imagine your basic factory:

factory :project

Before long you’re adding relations to projects, and the first thing people do is this:

factory :project do
  association :user
end

This immediately means that every single time you instantiate a project, you’re getting a user as well. In most cases this is more than you want, and if you continue to follow down this path you end up with a huge slow test suite. I prefer a slightly different strategy: start small and compose.

factory :project do
  ...
end

trait :with_manager do
  association :user, factory: :manager
end

This defines a very simple factory :project which gives you only a project and allows you to build a project with an associated user like so:

FactoryGirl.create(:project, :with_manager)

The result is a couple more arguments when you use the factory, but the overall code is more intention revealing.

If this is too much, you could always create a more descriptive factory:

factory :managed_project, parent: :project, traits: [:with_manager]

If you stick with this strategy, you’ll find that tests are more concise, factories are more useful and your test suite run time won’t grow as fast.