getters not working for refs

I found this issue as I wanted to do:

```
var schema = new Schema({
  payments: [
    {
      ref: 'Payment',
      type: ObjectId
    }
  ]
});

schema.virtual('supportedPayments').get(function() {
  var payments = this.getValue('payments');

  return payments.map(function(payment) {
    return payment.type;
  });
});

schema.path('payments').get(function(articles) {
  return articles.map(function(article) {
    return article.id;
  });
});
```