How to modify value in array by position at aggregation framework

Question

Let's say a have a simple document:

{
   array: ["a", "b", "c", "d"]
}

How to modify the second value in aggregation?

With update it is very simple:

db.collection.updateOne({},
   { $set: { "array.1": "B" } }
)

gives:

{
   array: ["a", "B", "c", "d"]
}

In aggregation framework you can use this one:

db.collection.aggregate([
   {
      $set: {
         "array": {
            $map: {
               input: "$array",
               in: {
                  $cond: {
                     if: { $eq: [{ $indexOfArray: ["$array", "$$this"] }, 1] },
                     then: "B",
                     else: "$$this"
                  }
               }
            }
         }
      }
   }
])

However, this fails when the array is not a Set, i.e. values are not unique like this one

{
   array: ["a", "b", "c", "b"]
}

Ashh · Accepted Answer · 2020-02-13 10:20:13Z

4

You can loop over the $size(length) of the array using $range and then get the index of the element and can change the value of the index element to whatever you want.

db.collection.aggregate([
  { "$project": {
    "array": {
      "$map": {
        "input": { "$range": [0, { "$size": "$array" }] },
        "in": {
          "$cond": [
            { "$eq": ["$$this", 1] },
            "B",
            { "$arrayElemAt": ["$array", "$$this"] }
          ]
        }
      }
    }
  }}
])

MongoPlayground

answered Feb 13, 2020 at 10:20

Ashh

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Comments

matthPen · Accepted Answer · 2020-02-13 10:39:28Z

1

Another solution is to $unwind your array with includeArrayIndex and preserveNullAndEmptyArrays, set value with condition, then group. But beware, although it can be useful in some situations, it can consume more server resources in some cases.

db.collection.aggregate([
  {
    $unwind: {
      path: "$array",
      includeArrayIndex: "index",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $addFields: {
      array: {
        $cond: {
          if: {
            $eq: [
              "$index",
              1
            ]
          },
          then: "B",
          else: "$array"
        }
      }
    }
  },
  {
    $group: {
      _id: "$_id",
      array: {
        $push: "$array"
      }
    }
  }
])

you can test here

answered Feb 13, 2020 at 10:39

matthPen

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1 Comment

Jean Monet Over a year ago

all this to modify 1 element in array :(

Wernfried Domscheit · Accepted Answer · 2021-11-20 14:19:03Z

I found a solution which requires less coding:

db.coll.aggregate([
   {
      $set: {
         array: {
            $function: {
               body: function (array, i, val) { array[i] = val; return array; },
               args: ["$array", 1, "B"],
               lang: "js"
            }
         }
      }
   }
])

When you need to update nested array, then the difference is even bigger. Let's assume a nested array as this:

{
   action: [
      { task: ['a', 'b', 'c', 'd'] },
      { task: ['aa', 'bb', 'cc', 'dd'] },
      { task: ['aaa', 'bbb', 'ccc', 'ddd'] }
   ]
}

It can be updated simply with

db.collection.aggregate([
   {
      $set: {
         action: {
            $function: {
               body: function (action, a, t, val) { action[a].task[t] = val; return action; },
               args: ["$action", 1, 2, "new CC"],
               lang: "js"
            }
         }
      }
   }
])

compared to

let arrayIndex = 1
let taskIndex = 2
db.collection.aggregate([
     {
        $set: {
           action: {
              $map: {
                 input: { $range: [0, { $size: "$action" }] },
                 as: "a",
                 in: {
                    $cond: {
                       if: { $eq: ["$$a", arrayIndex] },
                       then: {
                          $let: {
                             vars: { action: { $arrayElemAt: ["$action", "$$a"] } },
                             in: {
                                $mergeObjects: ["$$action", {
                                   task: {
                                      $map: {
                                         input: { $range: [0, { $size: "$$action.task" }] },
                                         as: "t",
                                         in: {
                                            $cond: {
                                               if: { $eq: ["$$t", taskIndex] },
                                               then: "new CC",
                                               else: { $arrayElemAt: ["$$action.task", "$$t"] }
                                            }
                                         }
                                      }
                                   }
                                }]
                             }
                          }
                       },
                       else: { $arrayElemAt: ["$action", "$$a"] }
                    }
                 }
              }
           }
        }
     }
 ])

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