title: Eager load polymorphic
body: <p>Using Rails 3.2, what's wrong with this code?</p>
  
  <pre><code>@reviews = @user.reviews.includes(:user, :reviewable)
  .where('reviewable_type = ? AND reviewable.shop_type = ?', 'Shop', 'cafe')
  </code></pre>
  
  <p>It raises this error:</p>
  
  <blockquote>
    <p>Can not eagerly load the polymorphic association :reviewable</p>
  </blockquote>
  
  <p>If I remove the <code>reviewable.shop_type = ?</code> condition, it works.</p>
  
  <p>How can I filter based on the <code>reviewable_type</code> and <code>reviewable.shop_type</code> (which is actually <code>shop.shop_type</code>)?</p>