title: Populate a mongoose model with a field that isn't an id
body: <p>Is it possible to populate a mongoose model with a field of a reference model that isn't the _id ... e.g. a username.</p>
  
  <p>so something like</p>
  
  <pre><code>var personSchema = Schema({
    _id     : Number,
    name    : String,
    age     : Number,
    stories : { type: String, field: "username", ref: 'Story' }
  });
  </code></pre>