How to flatten a subdocument into root level in MongoDB?

Question

For example, if I have a document like this

{
  a: 1,
  subdoc: {
          b: 2,
          c: 3
          }
}

How can I convert it into a

Answer 1

We can do this with $replaceWith an alias for $replaceRoot and the $mergeObjects operator.

let pipeline = [
   {
      "$replaceWith": {
         "$mergeObjects": [ { "a": "$a" }, "$subdoc" ]
      }
   }
];

or

let pipeline = [
    {
        "$replaceRoot": {
            "newRoot": {
                "$mergeObjects": [{ "a": "$a" }, "$subdoc" ]
            }
        }
    }
];

db.collection.aggregate(pipeline)