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

[steevej (Steeve Juneau)]

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

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

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.