Convert array of objects to string

I am having a MongoDB aggregate result:

{
    "healthDetails": [
        {
          "problem": "Fever",
          "note": "normal"
        },
        {
          "problem": "Allergy",
          "note": "critical"
        }
      ]
}

I need to convert the above array like this:

{
    "healthDetails": "Fever - normal, Allergy - critical"
}

Need to remove square brackets and double quotes and concat the values by comma-separated.

How to do that in MongoDB projection?

Need some valuable help.

>Solution :

Work with aggregation pipeline.

1. $reduce – Iterate the elements in the healthDetails array and transform into the string by concatenating the value with $concat.

2. $trim – Remove the beginning and ending characters for ", " from the result generated in 1.

db.collection.aggregate([
  {
    $set: {
      "healthDetails": {
        $trim: {
          input: {
            $reduce: {
              input: "$healthDetails",
              initialValue: "",
              in: {
                $concat: [
                  "$$value",
                  ", ",
                  {
                    $concat: [
                      "$$this.problem",
                      " - ",
                      "$$this.note"
                    ]
                  }
                ]
              }
            }
          },
          chars: ", "
        }
      }
    }
  }
])

Demo @ Mongo Playground