title: Is it possible to find by expression using the mongo find function?
body: <p>Say I have data like this:</p>
  
  <pre><code>{
    a: 2,
    b: 1
  }
  </code></pre>
  
  <p>I want to find all the values where a is greater than b.</p>
  
  <p>I can obviously do this using the <strong>aggregate pipeline</strong>, or using <strong>$where</strong>, but can I do it using the plain old find function? It feels like it ought to be possible to do this, as I would in the aggregate pipeline:</p>
  
  <pre><code>db.collection.find({
    a: {
      '$gt': '$b'
    }
  })
  </code></pre>
  
  <p><em>(this doesn't work)</em></p>
  
  <p>Is this not supported, and if not, is this by design, philosophy or omission? Alternately, am I just being an idiot?</p>