How to emit an array of values using mapReduce in mongodb?

Question

I am trying to write a mapReduce query to find the car with the lowest value of price plus mileage for each manufacturer. I am fairly new to this and am not sure how to emit a pair of values(i.e Milage & Price) then reduce them to a single value.

var mapFunction = function () {
  var key = this._id;
  var values = [this.Milage, this.Price];
  emit(key, values);
};

//Reducing failes here!
var reduceFunction = function (key, values) {
  let reducedValue = 0;
  values.forEach((num) => {
    reducedValue += parseFloat(num);
  });
  return reducedValue;
};

lowestValuePerManufacturer = async () => {
  const client = await MongoClient.connect(url, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
  }).catch((err) => console.log("Could not connect to MongoDB", err));

  try {
    const db = client.db(dbName);
    const collection = db.collection(collectionName);
    //Query starts here!
    const res = await collection.mapReduce(mapFunction, reduceFunction, {
      out: { inline: 1 },
    });
    console.log(res);
  } catch (err) {
   ///
  }
};

Data is an array of car objects as

{
    "Manufacturer": "Fiat",
    "Colour": "Black",
    "Model": "Panda",
    "Milage": 33292.0,
    "Price": 2668.32,
    "Classification": "Motor Cars",
    "Extras": [
        "SatNav",
        "ABS"
    ]
},

Hi. Why do you need map/reduce? when you can group the data by manufacturer and find the lowest price. As you can see in official documentation, MongoDB encourages you to use aggregation pipeline with $group instead of map/reduce: docs.mongodb.com/manual/core/map-reduce — Dheemanth Bhat
– Dheemanth Bhat, Commented Apr 7, 2021 at 12:45
Hello @DheemanthBhat I did go through the same docs I was going to try and use $group after this. Can you give me insights on how to use aggregation pipeline to achieve what I'm trying? — Rahul
– Rahul, Commented Apr 7, 2021 at 13:25

Dheemanth Bhat · Accepted Answer · 2021-04-07 19:19:39Z

1

So the idea is using aggregation:

First group the documents by Manufacturer using $group operator.
Then find minimum value for the sum of Price and Milage using $min operator.

Try this query:

db.cars.aggregate([
    {
        $group: {
            _id: "$Manufacturer",
            min_price_mileage: {
                $min: {
                    $add: ["$Price", "$Milage"]
                }
            }
        }
    },
    {
        $project: {
            _id: 0,
            Manufacturer: "$_id",
            min_price_mileage: "$min_price_mileage"
        }
    }
]);

Output:

/* 1 */
{
    "Manufacturer" : "Audi",
    "min_price_mileage" : 36020
},

/* 2 */
{
    "Manufacturer" : "Hyundai",
    "min_price_mileage" : 22083
}

Test data in cars collection:

[
    {
        "_id": ObjectId("..."),
        "Manufacturer": "Hyundai",
        "Colour": "Black",
        "Model": "Accent",
        "Milage": 29,
        "P15395ice": 1
    },
    {
        "_id": ObjectId("..."),
        "Manufacturer": "Hyundai",
        "Colour": "Red",
        "Model": "Venue",
        "Milage": 33,
        "Price": 22050
    },
    {
        "_id": ObjectId("..."),
        "Manufacturer": "Audi",
        "Colour": "Blue",
        "Model": "Q3",
        "Milage": 20,
        "Price": 36000
    },
    {
        "_id": ObjectId("..."),
        "Manufacturer": "Audi",
        "Colour": "Blue",
        "Model": "A4",
        "Milage": 31,
        "Price": 40900
    }
]

edited Apr 7, 2021 at 19:19

answered Apr 7, 2021 at 19:12

Dheemanth Bhat

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3 Comments

Rahul Over a year ago

Thank you! I'll still try to make an equivalent of this using map/reduce since it falls for my assignment. sheesh

Dheemanth Bhat Over a year ago

Check these two courses by MongoDB its free: 1. university.mongodb.com/courses/M001/about 2: university.mongodb.com/courses/M121/about

Rahul Over a year ago

yeah i figured something like this! Thank you again

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