MongoDB Exercise - Find the average score for each restaurant

Write a MongoDB query to find the average score for each restaurant.

Structure of 'restaurants' collection :

{
  "address": {
     "building": "1007",
     "coord": [ -73.856077, 40.848447 ],
     "street": "Morris Park Ave",
     "zipcode": "10462"
  },
  "borough": "Bronx",
  "cuisine": "Bakery",
  "grades": [
     { "date": { "$date": 1393804800000 }, "grade": "A", "score": 2 },
     { "date": { "$date": 1378857600000 }, "grade": "A", "score": 6 },
     { "date": { "$date": 1358985600000 }, "grade": "A", "score": 10 },
     { "date": { "$date": 1322006400000 }, "grade": "A", "score": 9 },
     { "date": { "$date": 1299715200000 }, "grade": "B", "score": 14 }
  ],
  "name": "Morris Park Bake Shop",
  "restaurant_id": "30075445"
}

Query:

db.restaurants.aggregate([{
    $unwind: "$grades"
  },
  {
    $group: {
      _id: "$name",
      avgScore: {
        $avg: "$grades.score"
      }
    }
  }
])

Output:

{ _id: "Buddy'S Burrito & Taco Bar", avgScore: 13.333333333333334 },
{ _id: 'Bergen Beach Cafe', avgScore: 12 },
{ _id: 'Pergola Des Artistes', avgScore: 6.333333333333333 },
{ _id: 'ZumStammtisch', avgScore: 12.833333333333334 },
{ _id: 'Shubert Theater', avgScore: 10.4 },
{ _id: "Big Daddy'S Diner", avgScore: 13.8 },
{ _id: 'Strokos Gourmet Deli', avgScore: 12.8 },
{ _id: 'Azusa Of Japan', avgScore: 11 },
{ _id: 'Pax Wholesome Foods', avgScore: 6.875 },
{ _id: "Erin'S Isle", avgScore: 9.5 },

.....

Explanation:

The said query in MongoDB makes an array of documents, representing a restaurant and its average score. The _id field of each document will contain the name of the restaurant, and the avgScore field will contain the calculated average score.

The $unwind stage is used to "unwind" an array field grades in the restaurants collection. As a result, a separate document will be created for each grade in the restaurants collection.

The $group stage groups the documents by the name field and calculates the average score of all the grades for each restaurant using the $avg operator.

Note: This output is generated using MongoDB server version 3.6

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