title: Mongo: Ensuring latest nested attribute has a value between given arguments
body: <p>I have a mongo collection 'books'. Here's a typical book:</p>
  
  <pre><code>BOOK
  
  name: 'Test Book'
  author: 'Joe Bloggs'
  print_runs: [
    {publisher: 'OUP', year: 1981},
    {publisher: 'Penguin', year: 1987},
    {publisher: 'Harper-Collins', year: 1992}
  ]
  </code></pre>
  
  <p>I'd like to be able to filter books to return only books whose <em>last</em> print run was after a given date, and/or before a given date...and I've been struggling to find a feasible query. Any suggestions appreciated.</p>