title: Mongoose: How to query objects inside populate?
body: <p>I have this route for searching partial user input for <code>orderId</code>.</p>
  
  <pre><code>ordersAdminRouter.route('/searchorder/:term')
      .get(function(req, res){
          term = req.params.term;
          console.log(term);
          Orders.find({orderId: new RegExp(term)})
              .populate({ path: 'userPurchased products.product', select: '-username -password' })
              .exec(function(err, orders){
                  if (err) throw err;
                  console.log(orders);
  
                  res.json(orders);
              });
      });
  </code></pre>
  
  <p>Here is my schema</p>
  
  <pre><code>var orderSchema = new Schema({
      orderId: { type: String },
      userPurchased: { type: Schema.Types.ObjectId, ref: 'users' },
      products: [
          {
              product: { type: Schema.Types.ObjectId, ref: 'products' },
              size: { type: String, required: true },
              quantity: { type: Number, required: true },
              subTotal: { type: Number, required: true }
          }
      ],
      totalQuantity: { type: Number },
      totalPrice: { type: Number },
      modeOfPayment: { type: String },
      shippingAd: { type: String },
      refNumber: { type: String },
      isDone: { type: Boolean, default: false },
      orderStatus: { type: String, default: 'Pending' },
      dateOrdered: { type: Date, default: Date.now() },
      fromNow: { type: String }
  });
  </code></pre>
  
  <p>Now I need to search for the firstname and lastname inside userPurchased which I only get when I populate. How can search it?</p>