title: Mongo sort on array field
body: <p>I have a mongo collection with docs</p>
  
  <pre><code>   {a:1,b:[2,3,4]}
     {a:2,b:[1,5,6]}
     {a:3,b:[3,4,5]}
  </code></pre>
  
  <p>I want to sort by field b so that the result is </p>
  
  <pre><code>   {a:2,b:[1,5,6]}
     {a:3,b:[3,4,5]}
     {a:1,b:[2,3,4]}
  </code></pre>
  
  <p>Where arrays are sorted by highest elements in them.</p>
  
  <p>Any idea?</p>