Querying subdocuments within an array using MongoDB's aggregation framework

Currently, I'm facing a challenge while developing a statistics dashboard for a meditation app. I'm struggling with creating a MongoDB query to fetch the most popular meditations based on user progress. The key collections involved are users and meditations.

The structure of a user document in the users collection is as follows:

{
   "_id":{
      "$oid":"627b519f73b2bd3375f5a7a5"
   },
   "userProgress":{
      "meditationsPracticed":[
         {
            "timestamp":"11.5.2022, 9:03:34",
            "id":"5ef2ff0af1a23752be00651f"
         },
         {
            "timestamp":"11.5.2022, 12:46:03",
            "id":"5eca520c10fe0480d350c9a4"
         },
         /* additional meditation entries */
      ]
   }
}

On the other hand, a meditation document in the meditations collection is structured like this:

{
   "_id":{
      "$oid":"5eca520c10fe0480d350c9ac"
   },
   "name":"Sleep Well",
   "duration":{
      "$numberInt":"250"
   }
}

My aim is to devise a query that retrieves the most popular meditations based on the id values stored in the meditationsPracticed array within the userProgress field of the user document.

Any assistance in crafting this MongoDB query would be highly appreciated. Thank you!

I've experimented with multiple queries utilizing $unwind and $lookup but haven't achieved success

javascripttypescriptdatabasemongodbnosql

Answer №1

Start by $unwinding at the meditationsPracticed level. Use $group to calculate the count based on the meditations id. Apply $rank along with $setWindowFields to handle scenarios with multiple top meditations counts. Next, use $lookup to retrieve the top meditation entry from the meditations collection.

db.users.aggregate([
  {
    "$match": {
      "_id": {
        "$oid": "627b519f73b2bd3375f5a7a5"
      }
    }
  },
  {
    "$unwind": "$userProgress.meditationsPracticed"
  },
  {
    "$group": {
      "_id": {
        "$toObjectId": "$userProgress.meditationsPracticed.id"
      },
      "count": {
        "$sum": 1
      }
    }
  },
  {
    "$setWindowFields": {
      "partitionBy": "$_id",
      "sortBy": {
        "count": -1
      },
      "output": {
        "rank": {
          "$rank": {}
        }
      }
    }
  },
  {
    "$match": {
      "rank": 1
    }
  },
  {
    "$lookup": {
      "from": "meditations",
      "localField": "_id",
      "foreignField": "_id",
      "as": "meditationsLookup"
    }
  },
  {
    "$unwind": {
      "path": "$meditationsLookup",
      "preserveNullAndEmptyArrays": true
    }
  }
])

Check out Mongo Playground for reference!

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