Before we started

1. Kindly scan through the default get all query response body from [response.json]
2. Checkout [Tour Schema] for the schema type object
3. In this tutorial, we will trying to create some new fields by grouping and according based on their value in difficulty which will only contains easy, medium and difficult

Note: This controller function will be under a route call ‘/api/v1/tours/tour-stats’.

Code

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exports.getTourStat = async (req, res) => {
  try {
    console.log("get tour stat")
    const stats = await Tour.aggregate([
      {
        $match: { ratingsAverage: { $gte: 4.5 } }
      },
      {
        $group: {
          _id: '$difficulty', //everything in one group, _id:null, _id :{ $toUpper: '$difficulty' } => spells uppercase
          num: { $sum: 1 }, // find total of tours, aggreagate will loop through the DB data on by one, everytime it loop it will added 1 into this field
          numRatings: { $sum: '$ratingsQuantity' },
          avgRating: { $avg: '$ratingsAverage' },
          avePrice: { $avg: '$price' },
          minPrice: { $min: '$price' },
          maxPrice: { $max: '$price' },
        }
      },
      {
        $sort: { aveRating: 1 }
      },
    ])

    res.status(200).json({
      status: 'success',
      data: {
        stats: stats
      }
    });
  } catch (error) {
    res.status(400).json({
      status: 'fail',
      message: error
    })
  }
}

Result

{
    "status": "success",
    "data": {
        "stats": [
            {
                "_id": "difficult",
                "num": 2,
                "numRatings": 41,
                "avgRating": 4.6,
                "avePrice": 1997,
                "minPrice": 997,
                "maxPrice": 2997
            },
            {
                "_id": "medium",
                "num": 3,
                "numRatings": 70,
                "avgRating": 4.8,
                "avePrice": 1663.6666666666667,
                "minPrice": 497,
                "maxPrice": 2997
            },
            {
                "_id": "easy",
                "num": 8,
                "numRatings": 159,
                "avgRating": 4.5875,
                "avePrice": 661,
                "minPrice": 50,
                "maxPrice": 1997
            }
        ]
    }
}

Explanation

1. Aggregation Pipeline provides tons of operators just to achieve your desire goal. Check out [https://docs.mongodb.com/manual/reference/operator/aggregation/] for more built-in operators
2. First of all, we can use the operator $match to filter out the value according to the field we set. This case we only want to get the ratingsAverage is $gte greater or equal than 4.5
3. Next, we can create our own response field by grouping the existing field in the Database from specifying the field name inside _id.
   • The new field name will follow in this standard newFieldName : { operator : '$<FieldName> }
   • For example, We can use the operator $sum to count the total number of easy in difficulty field num: { $sum: 1 }.
   • When aggregation pipeline is running, it will then loop through the response and add the value assigned to the $sum operator. This case will be 1
4. We also uses the operator like $avg to get the Avarage number, $min to get the minimum value and $max to get the maximum value
5. We can the the operator $sort to sort the field we want. This case we already create our custom field (after $group operator), so it’s applicable to put inside the $sort operator as well. The concept is same as previously we did for Mongo.find query whereby 1 is sort ascending and -1 is sort descending