aggregation framework - 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 format like this? (without using project)

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

Answer 1

You can use MongoDB projection i.e $project aggregation framework pipeline operators as well. (recommended way). If you don't want to use project check this link

db.collection.aggregation([{$project{ . . }}]);

Below is the example for your case:

db.collectionName.aggregate
([
    { $project: { a: 1, 'b': '$subdoc.b', 'c': '$subdoc.c'} }
]);

Gives you the output as you expected i.e.

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