title: Mongoose: how to define a combination of fields to be unique?
body: <p>If I had a schema like this:</p>
  
  <pre class="lang-js prettyprint-override"><code>var person = new Schema({
    firstName:  String,
    lastName: String,
  });
  </code></pre>
  
  <p>I'd like to ensure that there is only one document with the same firstName and lastName.</p>
  
  <p>How can I accomplish this?</p>