title: mongoose with typegoose returns null for reference with valid objectId
body: <p>I am using the library <a href="https://github.com/szokodiakos/typegoose" rel="nofollow noreferrer">https://github.com/szokodiakos/typegoose</a> to create mongoose models.</p>
  
  <p>Item.ts</p>
  
  <pre><code>export class Item extends Typegoose { }
  
  const ItemModel = new Item().getModelForClass(Item);
  export default ItemModel;
  </code></pre>
  
  <p>User.ts</p>
  
  <pre><code>import { InventoryItem } from "./InventoryItem";
  
  export class BagItem extends Typegoose {
    @prop({ ref: Item })
    Item?: Ref&lt;Item&gt;;
  
    @prop()
    Quantity: Number;
  }
  
  export class User extends Typegoose {
    @arrayProp({ items: BagItem })
    Bag?: BagItem[];
  }
  
  export const BagItemModel = new BagItem().getModelForClass(BagItem);
  const UserModel = new User().getModelForClass(User);
  
  export default UserModel;
  </code></pre>
  
  <p>When i try to populate the Item, the item is null. But using the same database with regular mongoose models i am able to populate the Item field in BagItem.</p>
  
  <p>app.ts</p>
  
  <pre><code>UserModel.findById(req.user._id).then((user: any) =&gt; {
    return user.populate("Bag.Item").execPopulate();
  }).then((bagPopulatedUser: User) =&gt; {
    console.log("The bag", bagPopulatedUser.Cart);
  }).catch((err: MongoError) =&gt; {
    console.log(err);
  });
  </code></pre>