Get a Single Object rather than and Array with One Object in MongoDB aggration

Question

I have the following query from my Playground

Sample dataset:

[
  {
    "_id": {
      "$oid": "637742400b359f51cb95798d"
    },
    "learnerId": "6377422e8a43630dcfdfd410",
    "targetExam": "LON11PLUS",
    "subject": "VR",
    "sections": [
      {
        "name": "Verbal Reasoning",
        "timePerSection": {
          "$numberInt": "960"
        },
        "events": [
          {
            "$numberInt": "2329"
          },
          {
            "$numberInt": "2053"
          },
          {
            "$numberInt": "3486"
          },
          {
            "$numberInt": "3826"
          },
          {
            "$numberInt": "3336"
          },
          {
            "$numberInt": "2950"
          },
          {
            "$numberInt": "2009"
          },
          {
            "$numberInt": "4637"
          },
          {
            "$numberInt": "3308"
          },
          {
            "$numberInt": "2884"
          },
          {
            "$numberInt": "2072"
          },
          {
            "$numberInt": "3269"
          },
          {
            "$numberInt": "2498"
          },
          {
            "$numberInt": "2647"
          },
          {
            "$numberInt": "2619"
          },
          {
            "$numberInt": "3600"
          },
          {
            "$numberInt": "2283"
          },
          {
            "$numberInt": "3597"
          },
          {
            "$numberInt": "2419"
          },
          {
            "$numberInt": "1991"
          }
        ]
      }
    ],
    "created": "2022-11-18 08:28:48.105743",
    "session": {
      "id": "63778c23960b1e7e97353eea"
    },
    "completed": {
      "$date": {
        "$numberLong": "1668779074955"
      }
    },
    "score": {
      "$numberInt": "2"
    }
  },
  {
    "_id": {
      "$oid": "637742590b359f51cb957990"
    },
    "learnerId": "6377422e8a43630dcfdfd410",
    "targetExam": "LON11PLUS",
    "subject": "ENG",
    "sections": [
      {
        "name": "English",
        "timePerSection": {
          "$numberInt": "1500"
        },
        "events": [
          {
            "$numberInt": "779"
          },
          {
            "$numberInt": "785"
          },
          {
            "$numberInt": "791"
          },
          {
            "$numberInt": "786"
          },
          {
            "$numberInt": "784"
          },
          {
            "$numberInt": "792"
          },
          {
            "$numberInt": "783"
          },
          {
            "$numberInt": "795"
          },
          {
            "$numberInt": "781"
          },
          {
            "$numberInt": "3041"
          },
          {
            "$numberInt": "2053"
          },
          {
            "$numberInt": "3497"
          },
          {
            "$numberInt": "3840"
          },
          {
            "$numberInt": "3023"
          },
          {
            "$numberInt": "2285"
          },
          {
            "$numberInt": "3022"
          },
          {
            "$numberInt": "4644"
          },
          {
            "$numberInt": "2477"
          },
          {
            "$numberInt": "2472"
          },
          {
            "$numberInt": "3338"
          },
          {
            "$numberInt": "3270"
          },
          {
            "$numberInt": "2018"
          },
          {
            "$numberInt": "2288"
          },
          {
            "$numberInt": "2260"
          },
          {
            "$numberInt": "3603"
          },
          {
            "$numberInt": "2463"
          },
          {
            "$numberInt": "3600"
          },
          {
            "$numberInt": "3312"
          }
        ]
      }
    ],
    "created": "2022-11-18 08:29:13.804332",
    "session": {
      "id": "63777e1c960b1e7e97353d15"
    },
    "completed": {
      "$date": {
        "$numberLong": "1668775515232"
      }
    },
    "score": {
      "$numberInt": "6"
    }
  }
]

aggregate pipeline:

db.collection.aggregate([
  {
    $match: {
      learnerId: "6377422e8a43630dcfdfd410",
      targetExam: "LON11PLUS",
      completed: {
        $exists: true
      },
      
    },
    
  },
  {
    $sort: {
      completed: -1,
      
    },
    
  },
  {
    $group: {
      _id: "$subject",
      history: {
        $push: "$$ROOT",
        
      },
      
    },
    
  },
  {
    $group: {
      _id: null,
      data: {
        $push: {
          "k": "$_id",
          "v": "$history"
        },
        
      },
      
    },
    
  },
  {
    "$replaceRoot": {
      "newRoot": {
        "$arrayToObject": "$data"
      }
    }
  }
])

current output:

[
  {
    "ENG": [
      {
        "_id": ObjectId("637742590b359f51cb957990"),
        "completed": ISODate("2022-11-18T12:45:15.232Z"),
        "created": "2022-11-18 08:29:13.804332",
        "learnerId": "6377422e8a43630dcfdfd410",
        "score": 6,
        "sections": [
          {
            "events": [
              779,
              785,
              791,
              786,
              784,
              792,
              783,
              795,
              781,
              3041,
              2053,
              3497,
              3840,
              3023,
              2285,
              3022,
              4644,
              2477,
              2472,
              3338,
              3270,
              2018,
              2288,
              2260,
              3603,
              2463,
              3600,
              3312
            ],
            "name": "English",
            "timePerSection": 1500
          }
        ],
        "session": {
          "id": "63777e1c960b1e7e97353d15"
        },
        "subject": "ENG",
        "targetExam": "LON11PLUS"
      }
    ],
    "VR": [
      {
        "_id": ObjectId("637742400b359f51cb95798d"),
        "completed": ISODate("2022-11-18T13:44:34.955Z"),
        "created": "2022-11-18 08:28:48.105743",
        "learnerId": "6377422e8a43630dcfdfd410",
        "score": 2,
        "sections": [
          {
            "events": [
              2329,
              2053,
              3486,
              3826,
              3336,
              2950,
              2009,
              4637,
              3308,
              2884,
              2072,
              3269,
              2498,
              2647,
              2619,
              3600,
              2283,
              3597,
              2419,
              1991
            ],
            "name": "Verbal Reasoning",
            "timePerSection": 960
          }
        ],
        "session": {
          "id": "63778c23960b1e7e97353eea"
        },
        "subject": "VR",
        "targetExam": "LON11PLUS"
      }
    ]
  }
]

I get an array with a single object, while the way I have structured the query, there should always be a single object.

How can I get this single object as root rather than nested in an array?

No, the result is still "wrapped" in an array. I want a root object and nothing more. — Mike M
– Mike M, Commented Nov 21, 2022 at 11:23
The array you mean should be the aggregation output, which is by default an array. You can just get the first element from aggregation output. — ray
– ray, Commented Nov 21, 2022 at 11:26

Gobind Deep Singh · Accepted Answer · 2022-11-21 12:24:18Z

1

If I understand the question correctly, you want the aggregate method to return a single object and not an Array that contains one object.

If that understanding is correct, then it is worth mentioning that aggregate method always returns a cursor (ref) and as a rule of thumb in MongoDB, anything that returns a cursor (find, aggregate), will always return a list a.k.a. an array.

My apologies, if you did not want to hear this, but you cannot have an aggregation returning a single object instead of an array having a single object.

Your best bet here is to just hardcode the 0'th position to get the document. Something like:

let result = await db.users.aggregate([...]);
let my_user_aggregated_data = result[0];

I hope it clarifies.

answered Nov 21, 2022 at 12:24

Gobind Deep Singh

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2 Comments

Mike M Over a year ago

Yes, that clarifies it a lot. So, from what I understand "it cannot be done"

Gobind Deep Singh Over a year ago

Unfortunately, yes that's correct.

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