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

You can use $replaceRoot with a $addFields stage as follows:

db.collection.aggregate([
    { "$addFields": { "subdoc.a": "$a" } },
    { "$replaceRoot": { "newRoot": "$subdoc" }  }
])