Node.js GraphQL MongoDB Mongoose填充字段未获取多个类别

我一共用了GraphQL MongoDB Mongoose个，我有两个Collection ..users和categories，如下所示.

Category.js个

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

const categorySchema = new mongoose.Schema({
    title:{
        type: String,
        required: true
    },
    userid: {
        type: Schema.Types.ObjectId,
        ref: 'User',
        required: true,        
    },
    
    
});

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

说

User.js个

const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const userSchema = new mongoose.Schema({
    name:{
        type: String,
        required: true
    },
    email:{
        type: String,
        required: true,
        unique: true
    },
    password:{
        type: String,
        required: true
    }
    
});

module.exports = mongoose.model('User',userSchema);

Resolver.js

const User = require('../models/User');
const Category = require('../models/Category');

const resolvers = {
    
    getUsers: async () => {
        try {
            const users = await User.find().populate('categories');            
            return users;
        } catch(err){
            throw new Error(err);            
        }
    },
    

    
};

module.exports = resolvers;

如你所见..我有两个Collection .而在Categories个系列中..我正在将我的数据与userid相加.一切都很好..但我无法得到categories，因为它总是显示null作为回应.

下面是我的GraphQL查询.

query{
  getUsers{
    id,
    name
  }
}

这是我得到的回应，即使我有来自users个收集的userid个相同的数据.

{
  "data": {
    "getUsers": [
      {
        "id": "65d316bdab8179475bdb0fef",
        "name": "mittul",
        
      }
    ]
  }
}

请向我们报到..

得到这样的回应.

{
  "data": {
    "getUsers": null
  }
}



 try {
                var resources = {};

            const users = await User.aggregate([{
                    $group : {_id : null, totalPop: { $sum: "$pop" }}
                },{
                    $lookup: {
                        from: "Category", // from collection name
                        localField: "userid",
                        foreignField: "_id",
                        as: "categories"
                    }
                }]);
                console.log(users);
        } catch(err){
            console.log(err);
            throw new Error(err);            
        }

我也试过下面的代码，但它给了我null的响应，在console.log没有.

  User.find().populate("categories")
        .then(users => {
            console.log(users);
            //res.send(users);
        }).catch(err => {
                    
            throw new Error(err);            
        });

以下是我的两个Collection .

有没有人能给我指点一下我在这里错过了什么？

谢谢