Update element of an array if and only if data for element is present in new document mongodb

This is actually a pain. But there are a few options:

1 – Bulk operation

Your actions are count be simply a $pull (to remove the existing value from the array, IF it exist) and $push (add the correct value). You could do this through the bulk operations api to ensure that the writes are as atomic as possible:

const bulkOperation = db.students.initializeOrderedBulkOp()
bulkOperation.find({_id:11}).updateOne({$pull: {marksObtained: { chemistry: { $exists: true } } } })
bulkOperation.find({_id:11}).updateOne({$push: {marksObtained: { chemistry: 12 } } })
bulkOperation.execute()

eg: Dynamically in code:

const _idToUpdate = 11
const updateFields = [{ chemistry: 13 }, { zoology: 12 }]

const bulkOperation = db.collection('students').initializeOrderedBulkOp()
for (const field of updateFields) {
  for (const key in field) {
    const value = field[key]

    const pullOp = { $pull: { marksObtained: { } } }
    pullOp.$pull.marksObtained[key] = { $exists: true }

    const pushOp = { $push: { marksObtained: { } } }
    pushOp.$push.marksObtained[key] = value

    console.log('name', key, 'value', value, 'pullOp', JSON.stringify(pullOp), 'pushOp', JSON.stringify(pushOp))

    bulkOperation.find({ _id: _idToUpdate }).updateOne(pullOp)
    bulkOperation.find({ _id: _idToUpdate }).updateOne(pushOp)
  }
}
bulkOperation.execute()

2 – Custom $function to run javascript to set the values

As per this playground: https://mongoplayground.net/p/PcV6dMoyJ6Y.

You can set a custom javascript function to do the dirty work for you.

Annoyingly in playground, it doesn’t format very well, hence it being on a single line, but an example is this:

db.collection.update({
  _id: 11
},
[
  {
    $set: {
      marksObtained: {
        $function: {
          body: "function(marks) { const updateFields = { chemistry: 13, zoology: 12, french: 11 }; for (const updateFieldName in updateFields) { let updated = false;for (const markObtained of marks) { for (const markObtainedKey in markObtained) { if (markObtainedKey === updateFieldName) { markObtained[markObtainedKey] = updateFields[updateFieldName]; updated = true; } } } if (!updated) { const newTopic = {}; newTopic[updateFieldName] = updateFields[updateFieldName]; marks.push(newTopic); } } return marks }",
          args: [
            "$marksObtained"
          ],
          lang: "js"
        }
      }
    }
  }
])

Where the function is:

const updateFields = { chemistry: 13, zoology: 12, french: 11 }
for (const updateFieldName in updateFields) {
  let updated = false
  for (const markObtained of marks) {
    for (const markObtainedKey in markObtained) {
      if (markObtainedKey === updateFieldName) {
        markObtained[markObtainedKey] = updateFields[updateFieldName]
        updated = true
      }
    }
  }
  if (!updated) {
    const newTopic = {}
    newTopic[updateFieldName] = updateFields[updateFieldName]
    marks.push(newTopic)
  }
}
return marks

Obviously you’ll have to set the values as per a single object as I’ve done, but you adjust as you wish.

Note: It would be a lot easier if you either formatted your marksObtained as:

marksObtained: {
  physics: 10,
  chemistry: 12,
  maths: 11,
  biology: 9,
  social: 9
}

or

marksObtained: [
  {lesson: 'physics', score: 10},
  {lesson: 'chemistry', score: 12},
  {lesson: 'maths', score: 11},
  {lesson: 'biology', score: 9},
  {lesson: 'social', score: 9}
]

Having an array with a single object doesn’t really make sense.