Mongodb filter an array based on another array element

Question

Lets say I have document like below

{
"_id" : 1,
"Quality" : [ 
        "HIGH", 
        "LOW", 
        "LOW", 
        "HIGH"
    ],
    "Pages" : [ 
        10, 
        10, 
        12, 
        17
    ]
}

I need result as

{
"_id" : 1,
"HIGH" : 27
"LOW" : 22
}

I need to create two attributes HIGH and LOW as per the Quality by summing their corresponding index position values of Pages array.

I though of using $filter and $arrayElemAt, but i could not get index position while filtering on Pages attribute

high : { $sum:{ $filter: { input: "$Pages", as: "noOfPages", cond: {"$eq":[{ $arrayElemAt: ["$Quality", **Need to pass index position of $pages while filtering**]}, "HIGH"]}}}}

Thanks In Advance.

Answer 1

Can achieve this with 2 aggregation stages.

First is to map the 2 arrays into a single array.

Second is filtering the single array to include only HIGH(/LOW), and reducing the filtered array into a single sum element.

[{$project: {
  "Quality": {
    $map: {
      input: {$range: [0, {$size: "$Quality"}]},
      as: "idx",
      in: {
        "Quality": { $arrayElemAt: ["$Quality", "$$idx"] },
        "Pages": { $arrayElemAt: ["$Pages", "$$idx"] }
      }
    }
  }
}}, {$project: {
  "HIGH": {
    $reduce: {
      input: {
        $filter: {
          input: "$Quality",
          as: "x",
          cond: {$eq: ["$$x.Quality", "HIGH"]}
        }
      },
      initialValue: 0,
      in: {
        $add: ["$$value", "$$this.Pages"]
      }
    }
  },
  "LOW": {
    $reduce: {
      input: {
        $filter: {
          input: "$Quality",
          as: "x",
          cond: {$eq: ["$$x.Quality", "LOW"]}
        }
      },
      initialValue: 0,
      in: {
        $add: ["$$value", "$$this.Pages"]
      }
    }
  }
}}]

Which gives exactly what you've asked for:

{
  _id: 1
  HIGH:27
  LOW:22
}

Answer 2

You can use below aggregation

db.collection.aggregate([
  { "$project": {
    "Quality": {
      "$map": {
        "input": { "$range": [0, { "$size": "$Quality" }] },
        "in": {
          "Quality": { "$arrayElemAt": ["$Quality", "$$this"] },
          "Pages": { "$arrayElemAt": ["$Pages", "$$this"] }
        }
      }
    }
  }},
  { "$project": {
    "newArrayField": {
      "$map": {
        "input": { "$setUnion": ["$Quality.Quality"] },
        "as": "m",
        "in": {
          "k": "$$m",
          "v": {
            "$filter": {
              "input": "$Quality",
              "as": "d",
              "cond": {
                "$eq": ["$$d.Quality", "$$m"]
              }
            }
          }
        }
      }
    }
  }},
  { "$project": {
    "d": {
      "$arrayToObject": {
        "$map": {
          "input": "$newArrayField",
          "in": {
            "k": "$$this.k",
            "v": { "$sum": "$$this.v.Pages" }
          }
        }
      }
    }
  }}
])

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