title: Reindex just one particular record with sunspot
body: <p>I have two models</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    has_many :posts
  
    searchable do
      text :post_titles
    end
  
    def post_titles
      posts.map &amp;:title
    end
  end
  
  class Post &lt; ActiveRecord::Base
    belongs_to :user
  end
  </code></pre>
  
  <p>the problem is that when I update title of the Post sunspot doesn't update index for related user and it is not searchable by new data. If I do <code>User.index</code> it solves problem but takes too much time. Are there any better solutions to update parent record index on child record change(like reindex just parent record and not all users)?</p>