title: Node Mongoose: Update many query with reference to current record
body: <p>I want to update one column of all rows collection by creating a concatnated string</p>
  
  <p>My Schema has a property fileName and I would like construct a new property url.</p>
  
  <p>Can I refernce the current row/record to build a query which would look something like this?</p>
  
  <pre><code>dbSchema.File.update({}, { $set: { url: '/view/images/'+ fileName}}, options, callback);
  </code></pre>
  
  <p>Obviously the above does not work because fileName is not in scope.. </p>
  
  <p>Whats the best way of achieving this?</p>