title: before_destroy and dependent destroy not firing
body: <p>I have the following model associations:</p>
  
  <pre><code>class Slider &lt; ActiveRecord::Base
    has_one :featured_happening, :as =&gt; :featured_item, :dependent =&gt; :destroy   
    before_destroy :destroy_featured_happening
    after_create :create_featured_happening
  end
  
  class FeaturedHappening &lt; ActiveRecord::Base
    belongs_to :featured_item, :polymorphic =&gt; true
  end
  </code></pre>
  
  <p>When I destroy a <code>Slider</code> object I thought the <code>dependent =&gt; :destroy</code> would automatically destroy the <code>featured_item</code> but it does not.</p>
  
  <p>Slider controller</p>
  
  <pre><code> def destroy    
      slider = Slider.find(params[:id])
      slider.delete
      render(:status =&gt; :ok, :nothing =&gt; true )
    end
  </code></pre>
  
  <p>So then I tried a callback with <code>before_destroy</code> to manually delete the <code>featured_item</code> when a slider object is destroyed and nothing is getting called.</p>
  
  <p>How can I get the <code>featured_item</code> to be deleted when I delete a slider object? Using Rails 3.2.</p>