MongoDB replace an array with a single string

Question

I am struggling with grouping data in mongoDB and I would like to know if there is any way to convert an array into a string? Unfortunately, the objects that were included in $lookup are displayed in the array instead of a single string.

my code:

 Answer.aggregate([
       {$lookup:
           {
             from: "flashcards",
             localField: "flashcard",
             foreignField: "_id",

             as: "flashcards"
           }
       },
       {$lookup:
           {
             from: "flashcardcollections",
             localField: "flashcards.collectionId",
             foreignField: "_id",

             as: "flashcardcollection"
           }
       },  
 
     
       {
           $group: {
               _id:    { collectionName:"$flashcardcollection.name", isPublic: "$flashcardcollection.isPublic", } ,
                
               Answers: {$sum: 1 },
   
               CorrectAnswers: { $sum: { $cond: [ { $eq: [ "$isCorrect", true ] }, 1, 0 ] } },
               WrongAnswers: { $sum: { $cond: [ { $eq: [ "$isCorrect", false ] }, 1, 0 ] } },
               ListOfAnswers: { $push:  {date:"$date", isCorrect: "$isCorrect", prompt: "$flashcards.prompt" }}
               }
            },
 
   ], function (err, results) {
    statisticsPerCollection.push(results)

       console.log("statisticPerCollection: ",JSON.stringify(statisticsPerCollection, null,2))

       }) 
}

the result:

  [
    {
      "_id": {
        "collectionName": [
          "MongoDBG"
        ],
        "isPublic": [
          true
        ]
      },
      "Answers": 11,
      "CorrectAnswers": 8,
      "WrongAnswers": 3,
      "ListOfAnswers": [
        {
          "date": "2021-03-07T18:47:00.575Z",
          "isCorrect": true,
          "prompt": [
            "MongoDbSize"
...

expectations: "collectionName": "MongoDBG" instead of "collectionName": [ "MongoDBG" ]

GitGitBoom · Accepted Answer · 2021-03-17 16:44:18Z

1

Since you're only looking up one collection, you can extract it from the array that $lookup outputs before grouping using an additional $addFields or $unwind stage. That way both collectionName and isPublic are formatted correctly.

  ...

  {$lookup:
      {
        from: "flashcardcollections",
        localField: "flashcards.collectionId",
        foreignField: "_id",

        as: "flashcardcollection"
      }
  },


  // Use $addFields
  {$addFields: {
    flashcardcollection: {$arrayElemAt: ['$flashcardcollection', 0]}
  }},

  // Or use $unwind 
  {$unwind: "$flashcardcollection"},


  {$group: {
     _id: {
       collectionName:"$flashcardcollection.name",
       isPublic: "$flashcardcollection.isPublic"
    },
    ...
  },

  ...

edited Mar 17, 2021 at 16:44

answered Mar 9, 2021 at 23:32

GitGitBoom

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