MongoDB: Access current value of document when adding element to array via $push

Question

I have collection MyCollection which basically consists of its _id and a string called comment. This collection should be bulk-updatable.

That's done like this:

   for (const obj of inputObjects) {
      bulkObjectsToWrite.push({
        updateOne: {
          filter: { _id: obj._id },
          update: {
            $set: {
              comment: obj.comment
            }
          }
        }
      })
    }
    await MyCollection.bulkWrite(bulkObjectsToWrite)

So far so good.

However, now the requirement is, that a commentHistory should be added which should look like [{"oldValue": "oldValueOfComment", "newValue": "newValueOfComment"}, ...]

I know I need to use $push for adding a new object to the commentHistory array. But how do I access the comment of the document updated right now, i.e. its current value?

I've tried

$push: {
              commentHistory: {
                newValue: obj.comment,
                oldValue: '$comment',
              },
},

but to no avail. The string $comment is added hard-coded, instead of the field being accessed.

(Using Mongoose 5.12.10 and Mongo 4.4.18)

Noel · Accepted Answer · 2023-01-17 03:12:16Z

1

You need to use update with aggregate pipeline.

db.collection.update({
  "key": 1
},
[
  {
    $set: {
      "comment": "New",
      "commentHistory": {
        "$concatArrays": [    //concatenate existing history array with new array entry
          "$commentHistory",
          [
            {
              "newValue": "New",
              "oldValue": "$comment"  //referencing the existing value
            }
          ]
        ]
      }
    }
  }
])

Demo

answered Jan 17, 2023 at 3:12

Noel

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2 Comments

cis Over a year ago

Thansk! But do you happen to know how to do that in Mongoose? According to this answer and its comments things seem not that easy: stackoverflow.com/a/66922686/2710714

Noel Over a year ago

Oh, i thought you can simply replace your query with this. No, I don't know if this works with Mongoose.

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