如何修复Mongoose中Cannot read properties of null (reading 'populated')错误？

我使用Mongoose和Node.js处理订单数据，需要对订单中的products字段做嵌套关联填充，但测试时部分数据正常运行，部分数据抛出如下错误。

代码示例 #

订单Schema（orderSchema） #

const { ObjectId } = mongoose.Schema.Types;
const orderSchema = new mongoose.Schema({
 name: {
   type: String,
   required: false
 },
 products: [
       {
         product: { type: ObjectId, ref: 'products', required: false },
         quantity: { type: Number, required: false, default: 0 },
         unit: { type: String, required: false, default: '' }
       }
     ]
},
{
  timestamps: true
});
const orderModel = mongoose.model('orders', orderSchema);

产品Schema（productSchema） #

const productSchema = new mongoose.Schema(
  {
    name: {
      type: String,
      required: true
    },    
   type: {
     type: String,
     required: true
   }
}, { timestamps: true });
const productModel = mongoose.model('products', productSchema);

关联填充查询代码 #

const result = await orderModel.find({}).populate({
        path: 'products',
        model: 'products',
        select: {
          quantity: 1,
          unit: 1,
          product: 1
        },
        populate: {
          path: 'product',
          model: 'products',
          select: { type: 1, name: 1 }
        }
      });
console.log('result', result);

数据库中的订单数据 #

[
  {
    "_id": {
      "$oid": "64e8b34b24bf0243bc87fa82"
    },
    "name": "ORDER 1",
    "products": [
      {
        "quantity": 100,
        "unit": "kg",
        "_id": {
          "$oid": "64e8d05a1be39f48906dec4b"
        },
        "product": {
          "$oid": "64e8b34724bf0243bc87fa2f"
        }
      }
    ],
    "createdAt": {
      "$date": {
        "$numberLong": "1692971851401"
      }
    },
    "updatedAt": {
      "$date": {
        "$numberLong": "1692979290410"
      }
    },
    "__v": 0
  }
]

错误信息 #

Cannot read properties of null (reading 'populated')

错误堆栈：

TypeError: Cannot read properties of null (reading 'populated')
at markArraySubdocsPopulated (C:\Users\user\OneDrive\Documents\backend\backendNodeJS -
Copie\node_modules\mongoose\lib\helpers\populate\markArraySubdocsPopulated.js:41:18)
at assignVals (C:\Users\user\OneDrive\Documents\backend\backendNodeJS -
Copie\node_modules\mongoose\lib\helpers\populate\assignVals.js:195:7)
at _assign (C:\Users\user\OneDrive\Documents\backend\backendNodeJS - Copie\node_modules\mongoose\lib\model.js:5119:3)
at _done (C:\Users\user\OneDrive\Documents\backend\backendNodeJS - Copie\node_modules\mongoose\lib\model.js:4931:9)
at _next (C:\Users\user\OneDrive\Documents\backend\backendNodeJS - Copie\node_modules\mongoose\lib\model.js:4919:7)
at C:\Users\user\OneDrive\Documents\backend\backendNodeJS - Copie\node_modules\mongoose\lib\model.js:5028:5
at C:\Users\user\OneDrive\Documents\backend\backendNodeJS -
Copie\node_modules\mongoose\lib\model.js:5353:18
at process.processTicksAndRejections (node:internal/process/task_queues:77:11)

问题原因与解决方案 #

问题根源 #

第一层populate中错误指定了model: 'products'，但products是订单文档的子文档数组，并非引用products模型的字段。Mongoose无法找到对应关联，处理空值或无效引用时触发报错。部分数据正常是因为这些订单的products.product引用的产品文档存在，而报错数据可能存在无效引用或空值。

修复后的查询代码 #

去掉第一层populate的model配置，直接填充子文档内的product字段：

// 简化写法，直接指定嵌套路径
const result = await orderModel.find({}).populate({
  path: 'products.product',
  select: 'type name'
});
console.log('result', result);

如果需要保留子文档的quantity、unit字段，可使用正确的嵌套populate写法：

const result = await orderModel.find({})
  .populate({
    path: 'products',
    select: 'quantity unit product',
    populate: {
      path: 'product',
      model: 'products',
      select: 'type name'
    }
  });
console.log('result', result);

额外优化建议 #

1. 检查数据库中订单的products.product字段，清理引用不存在产品ID的数据，或添加match过滤有效引用：

populate: {
  path: 'product',
  model: 'products',
  select: 'type name',
  match: { _id: { $exists: true } }
}

2. 确保代码中正确导入ObjectId类型，避免类型不匹配问题。

内容的提问来源于stack exchange，提问作者Dev M