MongoDB使用Mongoose populate关联查询数组ID遇CastError问题

我有一个Popular Mongoose模型，其中PopularPosts数组存储Post模型的ObjectId，存储操作正常，但GET请求时该数组只返回ID，尝试用populate关联查询获取完整Post详情时出现CastError。相关代码及错误信息如下：

Popular模型 #

const mongoose = require('mongoose');

const PopularSchema = new mongoose.Schema(
    {
        PopularPosts: [{
            type: mongoose.Schema.Types.ObjectId,
            ref: 'Post',
            required: false,
        }],
        createdAt: {
            type: Date,
            default: Date.now,
        }
    },
    { timestamps: true }
  );
  
module.exports = mongoose.model("Popular", PopularSchema);

Post模型 #

const mongoose = require('mongoose');

const PostSchema = new mongoose.Schema(
    {
        title: {
            type: String,
            required: true,
        },
        img: {
            type: String,
            required: true,
        },
        category: {
            type: mongoose.Schema.Types.ObjectId,
            ref: "Category",
        },
        desc: {
            type: String,
            required: false,
        },
        createdAt: {
            type: Date,
            default: Date.now,
        }
    },
    { timestamps: true }
  );
  
module.exports = mongoose.model("Post", PostSchema);

查询代码 #

const Popular = require('../../models/Popular');

class FindPopularPosts {
    async find(req, res) {
        const { id } = req.params;

        try {
            const popularPosts = await Popular.findById({id}).populate({
                path: 'PopularPosts',
                model: 'Post', 
            }).lean();

            return res.status(200).json(popularPosts);
        } catch (err) {
            return res.status(500).json(err);
        }
    }
} 

module.exports = new FindPopularPosts();

Postman错误信息 #

{
    "stringValue": "\"{ id: '63b5a895fbb93698183d75d2' }\"",
    "valueType": "Object",
    "kind": "ObjectId",
    "value": {
        "id": "63b5a895fbb93698183d75d2"
    },
    "path": "_id",
    "reason": {},
    "name": "CastError",
    "message": "Cast to ObjectId failed for value \"{ id: '63b5a895fbb93698183d75d2' }\" (type Object) at path \"_id\" for model \"Popular\""
}

问题解决 #

错误原因 #

Popular.findById({id})的参数传递错误：findById()方法接受的是单个ObjectId字符串值，而不是一个包含id属性的对象。你传入的{id}会被Mongoose当作要转换为ObjectId的值，自然触发CastError。

修正后的查询代码 #

将findById({id})改为findById(id)即可：

const Popular = require('../../models/Popular');

class FindPopularPosts {
    async find(req, res) {
        const { id } = req.params;

        try {
            const popularPosts = await Popular.findById(id).populate({
                path: 'PopularPosts',
                model: 'Post', 
            }).lean();

            return res.status(200).json(popularPosts);
        } catch (err) {
            return res.status(500).json(err);
        }
    }
} 

module.exports = new FindPopularPosts();

验证说明 #

修正后，Mongoose会正确将id参数当作ObjectId查询Popular文档，同时通过populate('PopularPosts')关联查询出对应的完整Post数据，返回的PopularPosts数组不再是单纯的ID，而是包含所有Post字段的对象。

内容的提问来源于stack exchange，提问作者Routfin