我在Stack Overflow上读过几个主题，都是同样的问题（How to populate in this case Mongoose），但我不知道为什么在我的情况下它根本不起作用。
我有一个用户模型：

const UserSchema = new Schema({
  login: String,
  email: String,
  password: String,
  purchases: [{
    item: {
      type: Schema.Types.ObjectId,
      ref: 'cart'
    },
    quantity: {
      type: Number,
      default: 1
    }
  }
})];

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注意购买它是一个对象数组（为什么是对象？因为我需要一个额外的字段'quantity'，所以我被告知在这里SO如何实现这一点），有额外的字段- quantity。我在user.purchases上使用了push方法来添加新的项目，到目前为止，它工作得很好。主要问题出现在我尝试填充'购买'数组中的项目时。我尝试了不同的方法：

User.findById(userId).populate({
        path: 'purchases',
        populate: {
          path: 'item',
          model: 'product'
        }
    })

    User.findById(userId).populate({
        path: 'purchases.item',
        model: 'product',
    })
with and without 'model', was using populate('purchases') and etc., but everything was of no use...I alway get data with _id's of items instead of items data itself.

    {
        "_id": "5b60620689f0872a3010b726",
        "login": "lalal",
        "email": "ololo",
        "password": "hahahah",
        "purchases": [
            {
                "quantity": 1,
                "_id": "5b4b80c73c566508146b563f"
            },
            {
                "quantity": 1,
                "_id": "5b4b80c73c566508146b5640"
            },
            {
                "quantity": 1,
                "_id": "5b4b80c73c566508146b5643"
            }
        ],
        "__v": 3
    }

What I'm doing wrong, or maybe I made a mistake with a schema itself?

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UPD.：我做了单文件测试，结果相同，但现在整个代码在这里。有人能修复或解释这段代码的错误吗？：

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

const UserSchema = new Schema({
    name: String,
    age: Number,
    items: [{
        buy: {
            type: mongoose.Schema.Types.ObjectId,
            ref: 'product'
        },
        quantity: {
            type: Number,
            default: 1
        }
    }]
});

const ProductSchema = new Schema({
    title: String,
    price: Number
});

const Product = module.exports.product = mongoose.model('product', ProductSchema);
const User = module.exports.user = mongoose.model('user', UserSchema);

const newProduct = new Product({
    title: 'Ferrari',
    price: 10000
});

const newUser = new User({
    name: 'Jim',
    age: 20
});

newUser.items.push(newProduct);
newUser.save().then(async () => {
    const data = await User.findOne({name: 'Jim'})
        .then(user => {
            return user.populate({
                path: 'items',
                populate: {
                    path: 'buy',
                    model: 'product'
                }
            });
        });
    console.log(data);
});

mongoose.connect('mongodb://127.0.0.1:27017/megatest')
    .then(() => {
        mongoose.connection.collections.users.drop();
    });

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