MongoDB query with Mongoose - Matching subdocument attribute within an array

Question

I am trying to solve an issue with the format of documents in a MongoDB depicted below.

{
  "_id": {
    "$oid": "68dc4f78e11553b3647231e2"
  },
  "name": "Dutch Bros",
  "address": "25 Down Street, Salt Lake City, UT",
  "facilities": [
    "Hot drinks",
    "Food"
  ],
  "coords": [
    -0.9690884,
    51.455041
  ],
  "reviews": [
    {
      "author": "John Smith",
      "rating": {
        "$numberDecimal": "5"
      },
      "createdOn": {
        "$date": "2025-09-30T06:00:00.000Z"
      },
      "reviewText": "Great Place. Would come back again!"
    },
    {
      "author": "Jane Doe",
      "rating": {
        "$numberDecimal": "4"
      },
      "createdOn": {
        "$date": "2025-09-30T06:00:00.000Z"
      },
      "reviewText": "Okay Place. Would come back again!"
    }
  ]
}

My goal is to match and return only review sub-document(s) based on an attribute it has such as ratings being equal to five. I know that I could take the reviews array as a whole and iterate over them to search the attributes for what I need, but I want to be able to find it directly in the MongoDB query so the MongoDB server does the work while it saves my express server any additional workload.

I guess what I am asking is how do I reference the iterable objects in the reviews array. I hope that was clear enough and I apologize if it wasn't!

I've successfully implemented a version that iterates in a for loop on the express server in the controller, but this is what I want to avoid. I've also tried using a .get() method on the returned array because I saw that it was an available method for the object, but that yielded an error. I've also tried to build a query which was close to what I believe the solution is going to be but I can't understand how MongoDB element matches an array item, see below for my failed attempt.

const monReview = await Loc
            .findById(req.params.locationid)
            .select('name reviews')
            .where('reviews')
            .elemMatch({$eq: req.params.reviewRating})
            .exec();

Cade Bray · Accepted Answer · 2025-10-01 16:12:33Z

Took me a moment but I figured out that I needed to use an aggregate instead.

var monReview = await Loc
            .aggregate([
                {$match: {_id: new mongoose.Types.ObjectId(req.params.locationid)}},
                {
                    $project: {
                        name: 1,
                        reviews: {
                            $filter: {
                                input: "$reviews",
                                as: "review",
                                cond: {$eq: ["$$review.rating", req.params.reviewRating]}
                            }
                        }
                    }
                }
            ]);

This now matches documents on the locationid and creates a projection for name and reviews as I had wanted. What actually achieves the goal is in the projection I filter the reviews based on the condition of being equal to the parameter passed for rating.

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