title: Elasticsearch querying multiple indices while limiting the fields of one index
body: <p>I have an <code>Autocomplete</code> class with a results method that queries two different models to return data. It partially works great, but with one major issue. </p>
  
  <pre><code>  def results
      Searchkick.search @query, index_name: [Location, DiveCenter]
    end
  </code></pre>
  
  <p>I need to limit the hits returned from the index <code>DiveCenter</code> because of the way I have the index set up. It's set up this way because a user will either click on a <code>DiveCenter</code> and be taken to the <code>DiveCenter</code> show page OR click on a location and be taken to <code>/search?(city | country)=value</code>. </p>
  
  <pre><code>  searchkick searchable: [:name, :city, :state, :country]
    def search_data
      {
        name: name,
        city: location.city,
        state: location.state,
        country: location.country
      }
    end
  </code></pre>
  
  <p>So basically, what I need is this:</p>
  
  <pre><code>  def results
      Searchkick.search @query, index_name: [Location, DiveCenter], fields: ['location.city', 'location.state', 'location.country', 'dive_center.name']
    end
  </code></pre>