title: Update MongoDB field using value of another field
body: <p>In MongoDB, is it possible to update the value of a field using the value from another field?  The equivalent SQL would be something like:</p>
  
  <pre><code>UPDATE Person SET Name = FirstName + ' ' + LastName
  </code></pre>
  
  <p>And the MongoDB pseudo-code would be:</p>
  
  <pre><code>db.person.update( {}, { $set : { name : firstName + ' ' + lastName } );
  </code></pre>