title: Search by prefix on concatenation of fields
body: <p>Did not find an answer for this on Google.</p>
  
  <p>I have json objects, which contains <code>first_name</code> and <code>last_name</code>.</p>
  
  <p>I want to run search that will match if the query is prefix of <code>first_name + last_name</code> or <code>last_name + first_name</code>.</p>
  
  <p>E.g. for objects:</p>
  
  <pre><code>{
       "first_name":"Leo",
       "last_name": "messi"
  }
  {
       "first_name":"Leo",
       "last_name": "Cohen"
  }
  </code></pre>
  
  <ul>
  <li>leo -> should match both </li>
  <li>leo m-> should match the first one </li>
  <li>messi l-> should match the first one</li>
  <li>le -> should match both</li>
  <li>co -> should match the second one</li>
  </ul>