title: How to add a has_many :through with polymorphic models?
body: <p>Given the models:</p>
  
  <pre><code>class User
    has_many :cars
    has_many :foods
  end
  
  class Car
    belongs_to :user
    has_many :colors, :as =&gt; :owner
  end
  
  class Food
    belongs_to :user
    has_many :colors, :as =&gt; :owner
  end
  
  class Color
    belongs_to :owner, :polymorphic =&gt; true   # This can be Food or Car
  end
  </code></pre>
  
  <p>How can I say that <code>User</code> has many <code>Color</code> through both <code>Car</code> and <code>Food</code> on rails 3? And what about Rails 2?</p>