title: Update a value in MongoDB based on its current value
body: <p>I want to do something like this, but the <code>this</code> keyword doesn't seem to be set inside the update statement.</p>
  
  <p><code>db.items.update({foo:…}, {$set: {bar: this.foo}}, false, true)</code></p>
  
  <p>Do I have to use <code>eval</code> to accomplish this?</p>