title: Single object in mongodb-collection [v 3.0.2]
body: <p>I'd like to have a collection (test) to contain only one element. Therefore I have a update-query set to an empty object:</p>
  
  <pre><code>db.test.find() -&gt; empty
  var mongoData = {"name": "Biff"};
  db.test.update({}, mongoData, {upsert:true}, cb);
  db.test.find() -&gt; {"name": "Biff", "_id":"1"}  
  
  var mongoData = {"name": "Buff"}; 
  db.test.update({}, mongoData, {upsert:true}, cb);
  db.test.find() -&gt; {"name": "Buff", "_id":"1"}   
  </code></pre>
  
  <p>Why isn't it this:</p>
  
  <pre><code>db.test.find() -&gt; empty
  var mongoData = {"name": "Biff"};
  db.test.update({}, mongoData, {upsert:true}, cb);
  db.test.find() -&gt; {"name": "Biff", "_id":"1"}   
  
  var mongoData = {"name": "Buff"};
  db.test.update({}, mongoData, {upsert:true}, cb);
  db.test.find() -&gt; [{"name": "Biff", "_id":"1"}, {"name": "Buff", "_id":"2"}]  
  </code></pre>
  
  <p>Updating an empty object '{}' seems to ensure that there isn't created another element in this collection. I thought, that {} couldn't be found, so {"name":"Biff"} gets created and a _id is set to it. If I try to update {} another time, it is not creating {"name":"Biff"} again with a different _id.
  It is overwriting the only element in the collection. 
  Why is this so?</p>