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"]
}

score 4 · Accepted Answer · answered Feb 13 '20 at 10:20

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

score 1 · Answer 2 · answered Feb 13 '20 at 10:39

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

all this to modify 1 element in array :( – Jean Monet Sep 19 '22 at 10:17 — Jean Monet, Sep 19 '22 at 10:17

Wernfried Domscheit · Answer 3 · 2021-11-20T14:19:03.513

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"] }
                    }
                 }
              }
           }
        }
     }
 ])

How to modify value in array by position at aggregation framework

3 Answers3

Linked