title: Has_many + has_many through parent of same class until root
body: <p>We've got a class called <code>Taxon</code>, a <code>taxon</code> will have a parent, which might have a parent, etc ... which eventually leads to a root taxon. </p>
  
  <p>I'm adding a relationship to <code>Taxon</code>: <code>OnboardingRequirement</code>, so that <code>Taxon</code> will <code>have_many :onboarding_requirements</code>. </p>
  
  <p>I want the relationship to also encompass its parent relationships and grandparents and great(^n)grandparents. </p>
  
  <p>Is it possible to compound a relationship? Is it possible to set taxon to <code>has_many :onboarding_requirements, through: :parent</code> <em>and</em> without the <code>through</code>?</p>
  
  <p>I know it would be possible to just loop through the parents and concat all <code>:onboarding_requirements</code> -- and the tree isn't terribly tall (4 or 5 at the deepest), but this seems like something that I should solve in the relationship, and not compute it at calltime.</p>
  
  <p>Using Rails 3.2.x, Ruby 2.1.x</p>