title: Get ID of affected document by using update()
body: <p>I'm using this for doing an upsert:</p>
  
  <pre><code>Articles.update(
      { title: title },
      { 
          title: title,
          parent: type
      },
      { upsert: true }, 
      function(res) {
          return console.log(res);
      }
  );
  
  console.log(needToGetID);
  </code></pre>
  
  <p>Now I need to get the _id of the document which has been updated or inserted. I thought I can get that via callback, but <code>res</code> is undefined.
  I assume there is just one unique document which is defined by the query.</p>
  
  <p><strong>Update</strong></p>
  
  <p>Forgot to mention that I'm using meteor...</p>