Is it possible to set a field in the last element in an array?

Working with Data
,
1

Is it possible to set a field in the last element in an array? I tried a couple of options but sadly did not work for me. (I’m using NodeJS)

I have added an example document at bottom in which chat is an array field with objects. Each object has question and answer fields. I want to update the document by setting answer in the last element, where answer will have the following shape:

{
    "id": "1234",
    "message": "First Question",
    "date": "2024-09-30T20:37:44.653Z"
}

I also want to make sure that answer field does not exist, and question field is present

{
  "_id": {
    "$oid": "66fb0ba517b93cb5134c9835"
  },
  "chat": [
    {
      "question": {
        "id": "1234",
        "message": "First Question",
        "date": "2024-09-30T20:37:44.653Z"
      },
      "answer": {
        "id": "abc",
        "message": "First Question",
        "date": "2024-09-30T20:37:44.653Z"
      }
    },
    {
      "question": {
        "id": "1234",
        "message": "First Question",
        "date": "2024-09-30T20:37:44.653Z"
      }
      // <-------- Add answer object to here
    }
  ]
}
2

It sounds like you’re actually trying to add a new element to the array rather than just updating the last element. If that’s the case, the correct approach would be to use the $push operator. This is the easiest way to append a new object to the end of an array in MongoDB.

const result = await collection.updateOne(
      { _id: documentId },
      { $push: { chat: newChat } } // Adds the new object to the end of the 'chat' array
    );

$push: Adds a new object to the end of the chat array.

3

What I understand is that you want the chat array to become:

That is you want the 2nd element, that is chat[ 1 ], to the answer object because the question object is present and the answer is not.

If that is the case, look at this playground. It uses the $ operator to update the element specified by $elemMatch to specify an element that has a question but no answer.

It will not be necessarily the last, in fact it is the first that matches the $elemMatch.