Express mongoose填充嵌套的子文档

我有下面的型号

var fieldsSchema = new Schema({
    name: String,
    type: String,
    value: String,
    media: [{ type: Schema.Types.ObjectId, ref: 'Upload' }],
    required: Boolean,
    recepientvisible: Boolean
})

var orderSchema = new Schema({
  number: String,
  date: Number,
  updated: Number,
  type: { type: Schema.Types.ObjectId, ref: 'OrderTemplate' },
  currentstatus: String,
  comment: String,
  assignedTo: { type: Schema.Types.ObjectId, ref: 'User' },
  createdBy: { type: Schema.Types.ObjectId, ref: 'User' },
  statuses: [{
    name: String,
    fields: [fieldsSchema]
  }]
});

var Order = mongoose.model('Order', orderSchema);

module.exports = Order;

我提出以下要求

app.post('/order/', function (req, res) {
Order.find()
           .populate({ path:'type', select: 'name -_id'})
           .populate({ path:'assignedTo', select: 'name -_id'})
           .populate({ path:'createdBy', select: 'name -_id'})
           .populate({ path:'statuses', populate: { path: 'fields', populate: { path: 'media'} }})
           .exec(function (err, orders) {
                                 if (err) throw err;
                                 res.send(orders)
                                });
  })

我需要的是填充媒体字段。但作为响应，我只得到了

\u id

的数组

所有其他字段都填充良好

如何正确填充媒体字段