title: has_many through and polymorphic relationships
body: <p>I don't know if this is possible, but here goes:</p>
  
  <pre><code>FruitBasket
    has_many :apples
    has_many :bananas
    ######## What to put here to access Worm through its pest_holder relationship?
  Apple
    has_many :worms, :as =&gt; :pest_holder
    belongs_to :fruit_basket
  Banana
    has_many :worms, :as =&gt; :pest_holder
    belongs_to :fruit_basket
  Worm
    belongs_to :pest_holder, :polymorphic =&gt; true
  </code></pre>
  
  <p>What is the relationship I need to be able to call:</p>
  
  <pre><code>red_delicious = Apple.first
  red_delicious.worms.whatever
  </code></pre>
  
  <p>And have it grab all of the Worm's through Apple's and Banana's polymorphic relationship with Worm?</p>
  
  <p>It seems kind of backwards, but I appreciate the help anyway! If there's any clarification needed, just ask.</p>