title: Specify trait for belongs_to assocation in Factory Girl
body: <p>I'm using Factory Girl, and I want to specify a belongs_to association that uses a trait. This code (without traits) works:</p>
  
  <pre><code>FactoryGirl.define do
    factory :challenge_participation do
      user
      challenge
      program
    end
  end
  </code></pre>
  
  <p>But how would I add the <code>with_days</code> trait to my program? (The <code>with_days</code> trait sets up some has_many associations).</p>
  
  <p>I imagine it'd be something like this, but I can't find the right syntax:</p>
  
  <pre><code>FactoryGirl.define do
    factory :challenge_participation do
      user
      challenge
      program :with_days # THIS DOESN'T WORK
    end
  end
  </code></pre>