Question 117805 in Mongodb Question posted in
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MongoDB Aggregation – Perform sum for the value in the objects with dynamic keys

Let’s say I have the following collection with _id and traits.

[
    {
      _id: 1,
      traits: {
        Rarity: {
          infoin: 15,
        },
        Type: {
          iron: 3,
          sliver: 5,
          wood: 7,
        },
      },
    },
    {
      _id: 2,
      traits: {
        Cloth: {
          barron1: 11,
          barron2: 12,
        },
        Hair: {
          black: 6,
          yellow: 9,
          red: 8
        }
      },
    },
    ...
]

As you can see keys of traits are dynamic and the keys of sub-objects as well.

Here is the result I wanna get:

[
    {
      _id: 1,
      traits: 15,
    },
    {
      _id: 2,
      traits: 23
    }
]

Tip:

infocoin = iron + sliver + wood

barron1 + barron2 = black + yellow + red

Tags:

2

Answers


  1. 0

    1. $set – Set traitObjs array field by converting the object to array via $objectToArray.

    2. $set – Set firstTraitValues field by getting the value of first document from traitObjs array, then converting from object to array via $objectToArray.

    3. $project – Decorate the output document. Set traits field by converting firstTraitValues array to number type with $reduce and $sum all the v values.

    db.collection.aggregate([
  {
    $set: {
      traitObjs: {
        $objectToArray: "$traits"
      }
    }
  },
  {
    $set: {
      firstTraitValues: {
        $objectToArray: {
          $first: "$traitObjs.v"
        }
      }
    }
  },
  {
    $project: {
      traits: {
        $reduce: {
          input: "$firstTraitValues",
          initialValue: 0,
          in: {
            $sum: [
              "$$value",
              "$$this.v"
            ]
          }
        }
      }
    }
  }
])

    Sample Mongo Playground

    Since all the values in the first key document and the second key document of traits are the same,

    infocoin = iron + sliver + wood

    barron1 + barron2 = black + yellow + red

    Hence the above approach just sums up all the values in the first key document of traits.

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  2. 0

    This answer is really the same as @yong-shun’s answer but it composes everthing into one "$project". I don’t know if it would be more efficient or not.

    db.collection.aggregate([
  {
    "$project": {
      "traits": {
        "$reduce": {
          "input": {
            "$objectToArray": {
              "$getField": {
                "field": "v",
                "input": { "$first": { "$objectToArray": "$traits" } }
              }
            }
          },
          "initialValue": 0,
          "in": { "$sum": [ "$$value", "$$this.v" ] }
        }
      }
    }
  }
])

    Try it on mongoplayground.net.

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