title: ElasticSearch search has_many relation
body: <p>I'm using ElasticSearch Rails and I'm trying to search through a has_many association. </p>
  
  <p>I wanna do this: <code>article.comments.search(...).</code></p>
  
  <p>However, it's not scoping to comments of that specific <code>article</code>. </p>
  
  <p>It's searching the entire <code>Comment</code> model, like this: <code>Comment.search(...)</code></p>
  
  <p>How do I scope this to a specific ActiveRecord <code>has_many</code> relation?</p>