尝试用 $avg 计算平均值并得到：参数必须是聚合管道运算符

Question

收到此错误：参数必须是聚合管道运算符这是将训练营作为 objectId 的课程“Schema”。

   const CourseSchema = new mongoose.Schema({

   bootcamp: {
        type: mongoose.Schema.ObjectId,
        ref: 'Bootcamp',
        required: true
    }
  });

聚合：

 //static method to get avg of course tuitions
    CourseSchema.statics.getAverageCost = async function (bootcampId) {
    console.log('calculating avg cost... with bootcampId:' + bootcampId);
    const obj = await this.aggragate([{
        $match: { bootcamp: bootcampId },
        $group: {
            _id: '$bootcamp',
            averageCost: { $avg: '$tuition' }
        }
    }]);
    console.log(obj);
    }

在保存或删除之前调用聚合：

...

// Call getAvarageCost after save
CourseSchema.post('save', function () {
    this.constructor.getAverageCost(this.bootcamp);
})

// Call getAvarageCost before remove
CourseSchema.post('remove', function () {
    this.constructor.getAverageCost(this.bootcamp);
})

...

Answer 1

$match 和 $group 必须在不同的管道操作中

const cursor = this.aggregate([
  { $match: { bootcamp: bootcampId } },
  {
    $group: {
      _id: '$bootcamp',
      averageCost: { $avg: '$tuition' },
    },
  },
])
console.log(await cursor.toArray())

尝试用 $avg 计算平均值并得到：参数必须是聚合管道运算符

trying to calc average with $avg and got: arguments must be aggregate pipeline operators

javascript

mongodb

mongoose

aggregate

aggregation