title: Query a subdocuments of a subdocument/list in Mongo
body: <p>Let's say we're interested in writing an extra slow HFT engine and are using Mongo.</p>
  
  <p>Here's the JSON, representing a bunch of daily forex objects:</p>
  
  <pre><code>{
      date: 2016-12-21,
      relation: {
          {currency: USD, value: 1.0},
          {currency: EUR, value: 0.9},
      }
  },
  {
      date: 2016-12-21,
      relation: {
          {currency: USD, value: 0.9},
          {currency: FJD, value: 1.0},
      }
  },
  </code></pre>
  
  <p>I want to fetch all daily forex objects where:</p>
  
  <ul>
  <li>date = 2016-12-21</li>
  <li>the forex object describes the FJD currency in at least one position of it's relation list</li>
  </ul>
  
  <p>How?</p>