title: Rails includes with scope
body: <p>I have a model called Author. An author has many Articles.
  Articles have a scope called .published that does: where(published: true).</p>
  
  <p>I want to load the author, with the published articles.
  I tried:</p>
  
  <pre><code>Author.includes(:articles.published).find(params[:author_id])
  </code></pre>
  
  <p>But that throws an error: undefined method 'published'.
  Any idea?</p>