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

I guess the easiest way is altering the result after it returns, using map.

collection.mapFunction = function(el) {
  el.b = el.subdoc.b;
  el.c = el.subdoc.c
  delete(el.subdoc);
  return el;
}

...

var result = await collection.aggregate(...).toArray();
result = result.map(collection.mapFunction);