mongoDB（在nodeJS中）聚合管道返回空数组

Question

I'm following Jonas Schmeddtman Node.js course and building a tour App. For some reason, when I send a request using postman on the route upon which this function is called, it returns an empty array instead of the manipulated data.

Below is my complete code. Thanks in advance.

      exports.getTourStats=async(req,res)=>
      {
        try
        {
          const stats= await Tour.aggregate([
            {
              $match: { ratingsAverage: { $gte: 4.5 } }
            },
            {
              $group:
              {
                _id: { $toUpper: '$difficulty' },
                      numTours: { $sum: 1 },
                      numRatings: { $sum: '$ratingsQuantity' },
                      avgRating: { $avg: '$ratingsAverage' },
                      avgPrice: { $avg: '$price' },
                      minPrice: { $min: '$price' },
                      maxPrice: { $max: '$price' }
              }
             
            }
          ]);
          
          res.status(200).json(
            {
              status:"success",
              data:
              {
                stats
              }
            });
        }
        catch(error)
        {
          res.status(400).json(
            {
              status:"failed!",
              message:error
            })
        }
      }

//an example document is as below.

    "id": 8,
    "name": "The Northern Lights",
    "duration": 3,
    "maxGroupSize": 12,
    "difficulty": "easy",
    "ratingsAverage": 4.9,
    "ratingsQuantity": 33,
    "price": 1497,
    "summary": "Enjoy the Northern Lights in one of the best places in the world",
    "description": "dummy description",
    "imageCover": "tour-9-cover.jpg",
    "images": ["tour-9-1.jpg", "tour-9-2.jpg", "tour-9-3.jpg"],
    "startDates": ["2021-12-16,10:00", "2022-01-16,10:00", "2022-12-12,10:00"]

---

// schema.

const tourSchema = new mongoose.Schema({
  name: {
    type: String,
    required: [true, 'A tour must have a name'],
    unique: true,
  },
  duration: {
    type: Number,
    required: [true, 'a tour must have a duaration'],
  },
  maxGroupSize: {
    type: Number,
    required: [true, 'a tour must have a max group size'],
  },
  difficulty: {
    type: String,
    required: [true, 'a tour must have a diffculty'],
  },
  ratingAverage: {
    type: Number,
    default: 4.5,
  },
  ratingQuantity: {
    type: Number,
    default: 0,
  },
  price: {
    type: Number,
    required: [true, 'A tour must have a price'],
  },
  priceDiscount: {
    type: Number,
  },
  summary: {
    type: String,
    trim: true,
    required: [true, 'A tour must have a summary'],
  },
  description: {
    type: String,
    trim: true,
  },
  imageCover: {
    type: String,
    required: [true, 'A tour must have an image cover'],
  },
  images: [String],
  createdAt: {
    type: Date,
    default: Date.now(),
    //to exclude the created at property from response sent back to user we put select property to false.
    select: false,
  },
  startDates: [Date],
});
//creating a model out of the schema we defined.
const Tour = mongoose.model('Tour', tourSchema);
module.exports = Tour;

Answer 1

In the end of aggregation you should use toArray() .Then only aggregated values get instead of getting the aggregationCursor as out.

  exports.getTourStats=async(req,res)=>
  {
    try
    {
      const stats= await Tour.aggregate([
        {
          $match: { ratingsAverage: { $gte: 4.5 } }
        },
        {
          $group:
          {
            _id: { $toUpper: '$difficulty' },
                  numTours: { $sum: 1 },
                  numRatings: { $sum: '$ratingsQuantity' },
                  avgRating: { $avg: '$ratingsAverage' },
                  avgPrice: { $avg: '$price' },
                  minPrice: { $min: '$price' },
                  maxPrice: { $max: '$price' }
          }
         
        }
      ]).toArray()
      
      res.status(200).json(
        {
          status:"success",
          data:
          {
            stats
          }
        });
    }
    catch(error)
    {
      res.status(400).json(
        {
          status:"failed!",
          message:error
        })
    }
  }