CODEWITHSUNDEEP
mongodb
Sanket hn
Sanket hn

Reputation: 79

How to calculate average rating of my product in mongodb?

This is the query I used to get the average of rating, but the result was null:

db.products.aggregate([{$unwind: "$reviews"}, {$group: {_id: "$reviews", ratingAvg: {$avg: "$rating"}}}])

This is my product schema:

    category: String,
    name: String,
    price: Number,
    image: String,
    description: String,
    stock: Number,
    reviews: [
        {
            type: mongoose.Schema.Types.ObjectID, ref: 'Review'
        }
})

This is my review schema:

    text: String,
    rating: Number,
    // author: {
    //  id: {type: mongoose.Schema.Types.ObjectID, ref: 'User'},
    //  name: String
    // }
    author: String
})

Every time I run this query I get:

{ "_id" : ObjectId("5f79d1b4b4b3c1061f1fbe52"), "ratingAvg" : null }
{ "_id" : ObjectId("5f79d1a5b4b3c1061f1fbe51"), "ratingAvg" : null }
{ "_id" : ObjectId("5f79d196b4b3c1061f1fbe4f"), "ratingAvg" : null }

It is showing ratingAvg as"null" whereas I have ratings for that product in my database.

Upvotes: 0

Views: 1980

Answers (1)

varman
varman

Reputation: 8894

$lookup helps to join two collections.

[
  {
    "$lookup": {
      "from": "Review",
      "localField": "reviews",
      "foreignField": "_id",
      "as": "reviews"
    }
  },
  {
    $unwind: "$reviews"
  },
  {
    $group: {
      _id: null,
      ratingAvg: {
        $avg: "$reviews.rating"
      }
    }
  }
]

Then you can easily find the average using $avg along with $group after destructure the array using $uniwnd

Working Mongo playground

Upvotes: 2

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