How to Convert the data structure of a column in mongo db and store it in another column?

There are 3 million rows in mongo db
I want to create a column B using column A.

A : [1,2,3,4]

# All level value is 1
B : [{"key" : 1, "level" : 1}, {"key" : 2, "level" : 1}, {"key" : 3, "level" : 1}. {"key" : 3, "level" : 1}]

What I have tried:

table.update_many({}, [
    {
        "$set": {
            "B": {"key" : "$A", "level" : 1}
        }
    }
])

But this resulted in, This is not I want.

B : {"sku_id" : [1,2,3,4], "level" : 1}

Is there any way to bulk update? or do I need to update one row at a time?

>Solution :

You need to use $map to iterate each element in the A array and transform into a new array with documents.

db.collection.update({},
[
  {
    "$set": {
      "B": {
        $map: {
          input: "$A",
          in: {
            "key": "$$this",
            "level": 1
          }
        }
      }
    }
  }
])

Demo @ Mongo Playground