title: Elasticsearch Rails Persistence Model: how to update nested objects?
body: <p>I'm starting with source docs such as the following:</p>
  
  <pre><code>"books" : [
    { 
      "title" : "book one", 
      "editor" : "me",
      "chapters" : [
        {
          "number" : "one",
          "author" : "first author"
        },
        {
          "number" : "two",
          "author" : "second author"
        }
      ]
    },
   ...
  ]
  </code></pre>
  
  <p>After importing my initial data, I want to add and index a field ('pdf' of type 'attachment') to each 'chapter'.   'pdf', as the name implies, is a pdf of the full chapter. </p>
  
  <p>How do I set this up with ElasticSearch persistence model? Specifically,
  - the mapping, and
  - the updating</p>