title: Upsert with $exists not allowed: The dollar ($) prefixed field is not valid for storage
body: <p>This Meteor server side (Mongo 1.1.18) tries to upsert a document according to the selector as shown but gives the following error:  </p>
  
  <pre><code>  myCol.upsert({name: 'sam', job: {$exists: false}}, {$set: {parents: ['jack', 'jacky']}});
  </code></pre>
  
  <blockquote>
    <p>MongoError: The dollar ($) prefixed field '$exists' in 'job.$exists' is not valid for storage.</p>
  </blockquote>
  
  <p>How can I upsert to this selected document? or create it if it does not exist? thx</p>