MongoDB Aggregate - Calculate total number of all nested array elements within an object

Question

I have a database structure which includes the following object:

"reactions": {
    "👊": [
        "5f3c569e70441a0017511fe2",
        "5f88d49dbf1b850017e28f4e",
        "5f488189b0c5970017aa8c7a"
    ],
    "😋": [
        "5f539086cfb52a00176089ed"
    ]
}

I want to be able to count the "number of reactions" (a total of all the nested elements) - so in this case, the expected result would be 4.

I tried the following, and this successfully counts the number of keys - the result is 2, but not the total number of values - which should be 4.

db.collection("posts").aggregate([{
    {
        $addFields: {
            reactionsArray: {
                $objectToArray: "$reactions"
            }
        }
    },
    {
        $addFields: {
            numberOfReactions: {
                $sum: {
                    $size: "$reactionsArray.v"
                }
            }
        }
    }
])

Answer 1

Use $reduce to sum size of each element of your array.

db.collection.aggregate([
  {
    $addFields: {
      reactionsArray: {
        $objectToArray: "$reactions"
      }
    }
  },
  {
    "$project": {
      reactionsSize: {
        $reduce: {
          input: "$reactionsArray",
          initialValue: 0,
          in: {
            "$add": [
              "$$value",
              {
                $size: "$$this.v"
              }
            ]
          }
        }
      },
      
    },
    
  }
])

MongoDB Playground

Answer 2

You have to $unwind reactionsArray and then use $group to get total count

db.collection.aggregate([
  {
    $addFields: {
      reactionsArray: {
        $objectToArray: "$reactions"
      }
    }
  },
  {
    $unwind: "$reactionsArray"
  },
  {
    $group: {
      _id:"$_id",
      numberOfReactions: {
        $sum: {
          $size: "$reactionsArray.v"
        }
      }
    }
  }
])

MongoDB Playground