How to create an association in FactoryGirl with a value lazy-evaluated from another FactoryGirl association

Is it possible to use an association as the value for another association in a trait?

Let me describe a (simplified) example. I have 3 models

• User
• Forum (belongs_to :forum)
• Thread (belongs_to :forum, belongs_to :user)

Please note that Thread contains an user_id, it does not belong to User through Forum (at the time being, I can't change such constraint).

What I'd like to know, is if there a way to define a trait like the one

FactoryGirl.define do
  factory :thread do
    trait :with_forum do
      association :user
      # Lazy-evaluate the value of user to the previously created factory
      # THIS IS THE KEY POINT: I want 'user' to be the factory created at the previous line
      association :forum, user: user
    end
  end
end

What the trait is supposed to do is to create an user and associate it to the thread. Then it should create a forum, but the user should be the same instance previously created.

There are mainly two reasons:

• The user associated with the forum is the same of the thread
• I don't want to create two user factories on cascade

Any idea? I tried to use the lazy evaluation, but no way to use it with associations.