title: Search condition for association
body: <p>Category has_many products.</p>
  
  <p>In category_index I defined:<br>
  <code>indexes :title</code><br>
  <code>has products(:id), :as =&gt; :product_ids</code></p>
  
  <p>In product_index I defined:<br>
  <code>indexes :title</code>  </p>
  
  <p>In searcher class:
  </p>
  
  <pre><code>product_ids = Product.search_for_ids('word', with: {user_id: 5})
  categories = Category.search('word')
  categories_where_products_match = Category.search(with: {product_ids: products_ids})
  </code></pre>
  
  <p>How can I merge <code>categories</code> and <code>categories_where_products_match</code> into one <code>ThinkingSphinx::Search</code> object?</p>