title: Rails/RSpec/FactoryGirl—How to make a field point to a method?
body: <p>I have this factory</p>
  
  <pre><code>FactoryGirl.define do
    factory :gst_report do
      association :organisation, factory: :active_organisation
      period_months 3
      period_start_date Date.parse('2015-01-01')
      period_end_date Date.parse('2015-02-01')
    end
  end
  </code></pre>
  
  <p>But in the tests <code>period_start_date</code> is often set to other dates. For each of these tests, I want <code>period_end_date</code> to simply be a month after <code>period_start_date</code>.</p>
  
  <p>Is it possible to make <code>period_end_date</code> a method that gets the current <code>period_start_date</code> and adds one month to it?</p>