Issue with MongoDB array aggregation and grouping by key in array of objects

Question

Considering an array like the one below which i'm trying to group by date and sum the durations:

[
  {
    "allDates": [
      {
        "duration": 153,
        "date": "2021-10"
      },
      {
        "duration": 20,
        "date": "2021-11"
      },
      {
        "duration": 181,
        "date": "2021-11"
      },
      {
        "duration": 180,
        "date": "2021-11"
      }
    ]
  }
]

I'm trying to achieve a sum of the duration but grouped by the dates. This is what i've tried so far:

db.collection.aggregate([
  {
    $addFields: {
      durations: {
        $arrayToObject: {
          $map: {
            input: "$allDates",
            as: "allDate",
            in: {
              k: {
                $toString: "$$allDate.date"
              },
              v: {
                $sum: {
                  $map: {
                    input: "$allDates",
                    as: "kv",
                    in: {
                      $cond: {
                        if: {
                          $eq: [
                            {
                              $toString: "$allDate.date"
                            },
                            {
                              $toString: "$$kv.k"
                            }
                          ]
                        },
                        then: "$$kv.duration",
                        else: 0
                      }
                    }
                  }
                }
              }
            }
          }
        }
      },
      
    }
  }
])

Unfortunately the result i get is:

[
  {
    "allDates": [
      {
        "date": "2021-10",
        "duration": 153
      },
      {
        "date": "2021-11",
        "duration": 20
      },
      {
        "date": "2021-11",
        "duration": 181
      },
      {
        "date": "2021-11",
        "duration": 180
      }
    ],
    "durations": {
      "2021-10": 534,
      "2021-11": 534
    }
  }
]

So it's adding them all up for every key instead of for each one separately, what am i missing here?

Basically I'm expecting to get:

...
"durations": {
      "2021-10": 153, 
      "2021-11": 381 
}

Answer 1

Try this one:

db.collection.aggregate([
  {
    $unwind: "$allDates"
  },
  {
    $group: {
      _id: "$allDates.date",
      duration: {
        $sum: "$allDates.duration"
      }
    }
  },
  {
    $group: {
      _id: null,
      durations: {
        $push: {
          k: "$_id",
          v: "$duration"
        }
      }
    }
  },
  {
    $project: {
      _id: 0,
      durations: {
        $arrayToObject: "$durations"
      }
    }
  }
])

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