Express中Mongoose查询Tweet时populate likes返回空数组问题

问题详情 #

我定义了如下两个Mongoose Schema：

Like模型Schema #

//create likes schema
const likes = mongoose.Schema({
    liked: {
        type: Boolean,
        default: false
    },
    tweet: {
        type: Schema.Types.ObjectId,
        ref: "Tweet"
    },
    author: {
        type: Schema.Types.ObjectId,
        ref: "User"
    }
});

module.exports = mongoose.model('Like', likes);

Tweet模型Schema #

const tweets = mongoose.Schema({
    content: {
        type: String,
        required: true,
    },
    author: {
        type: Schema.Types.ObjectId,
        ref: "User"
    },
    likes: [{
            type: Schema.Types.ObjectId,
            ref: "Like"
        }]
});

module.exports = mongoose.model('Tweet', tweets);

测试Like的populate查询时结果正常：

const likes = await Like.find().populate("author", "_id name email").populate("tweet", "_id content").exec()
res.json(likes)

返回数据：

[
    {
        "_id": "63921e53deb31c60249901e4",
        "liked": true,
        "tweet": {
            "_id": "63921e50deb31c60249901e1",
            "content": "tweet 1"
        },
        "author": {
            "_id": "63921e2ddeb31c60249901dd",
            "name": "Dave",
            "email": "something@gmail.com"
        },
        "createdAt": "2022-12-08T17:26:43.650Z",
        "updatedAt": "2022-12-08T17:26:43.650Z",
        "__v": 0
    }
]

但查询Tweet并populate likes字段时，得到的likes数组为空：

const tweets = await Tweet.find()
    .populate("author", "_id name email")
    .populate("likes", "_id")
    .sort({updatedAt: "desc"})
    .exec()

res.status(200).json(tweets)

返回数据：

[
    {
        "_id": "6393701aa62997f3454e81e1",
        "content": "My tweet",
        "author": "63936ffaa62997f3454e81dd",
        "likes": [],
        "createdAt": "2022-12-09T17:27:54.146Z",
        "updatedAt": "2022-12-09T17:27:54.146Z",
        "__v": 0
    }
]

实际Like集合中存在对应该Tweet的记录：

[
    {
        "_id": "63937140df6222756bd84ede",
        "liked": true,
        "tweet": {
            "_id": "6393701aa62997f3454e81e1",
            "content": "My tweet"
        },
        "author": {
            "_id": "63936ffaa62997f3454e81dd",
            "name": "Dave",
            "email": "admin@gmail.com"
        },
        "createdAt": "2022-12-09T17:32:48.251Z",
        "updatedAt": "2022-12-09T17:32:48.251Z",
        "__v": 0
    }
]

解决方案 #

问题核心是双向引用未同步更新：Mongoose不会自动维护跨文档的双向关联，你创建Like记录时仅在Like文档中关联了Tweet ID，但未将Like ID同步添加到对应Tweet的likes数组中，导致populate时无法匹配到关联数据。

修复步骤： #

1. 创建Like时同步更新Tweet
   新增Like记录后，手动更新对应Tweet的likes数组：

   // 创建Like示例
   const newLike = new Like({
       liked: true,
       tweet: tweetId, // 目标Tweet的ID
       author: userId
   });
   
   await newLike.save();
   
   // 同步更新Tweet的likes数组
   await Tweet.findByIdAndUpdate(
       tweetId,
       { $push: { likes: newLike._id } },
       { new: true } // 返回更新后的文档
   );
   

2. 补全已有数据的关联
   针对已存在的Like记录，批量同步到对应Tweet：

   const allLikes = await Like.find().populate('tweet');
   
   for (const like of allLikes) {
       if (like.tweet && !like.tweet.likes.includes(like._id)) {
           await Tweet.findByIdAndUpdate(
               like.tweet._id,
               { $push: { likes: like._id } }
           );
       }
   }
   

3. 可选：用中间件自动维护关联
   给Like模型添加post中间件，自动处理关联更新，避免手动操作：

   // 保存Like后同步更新Tweet
   likes.post('save', async function(doc) {
       await Tweet.findByIdAndUpdate(
           doc.tweet,
           { $addToSet: { likes: doc._id } }, // $addToSet避免重复添加
           { new: true }
       );
   });
   
   // 删除Like时从Tweet中移除关联
   likes.post('remove', async function(doc) {
       await Tweet.findByIdAndUpdate(
           doc.tweet,
           { $pull: { likes: doc._id } },
           { new: true }
       );
   });
   

完成以上操作后，重新执行Tweet的populate查询，即可正确获取到关联的Like记录。

内容的提问来源于stack exchange，提问作者WaqasRaza