如何在 mongoosastic 上填充？

问：

我的目标是在 mongoosastic 搜索中填充某些字段，但是如果我执行以下代码，它将始终返回

代码如下

var mongoose = require('mongoose');
var Category = require('./category');
var mongoosastic = require('mongoosastic');
var Schema = mongoose.Schema;

var ProductSchema = new Schema({
  category: { type: Schema.Types.ObjectId, ref: 'Category', es_schema: Category, es_indexed:true, es_select: 'name'},
  name: String,
  price: Number,
  image: String
});

ProductSchema.plugin(mongoosastic, {
  hosts: [
    'localhost:9200'
  ],
  populate: [
    {path: 'category', select: 'name'}
  ]
});

module.exports = mongoose.model('Product', ProductSchema);


var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var CategorySchema = new Schema({
  name: { type: String, unique: true, lowercase: true}
});

module.exports = mongoose.model('Category', CategorySchema);

api.js

router.post('/search', function(req, res, next) {
  console.log(req.body.search_term);
  Product.search({
    query_string: { query: req.body.search_term }
  }, function(err, results) {
    if (err) return next(err);
    res.json(results);
  });
});

我应该怎么做才能在 mongoosastic 中填充某个类别？