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
    }

Answer 2

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

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

Answer 3

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)

Answer 4

You can also use $$ROOT in $mergeObjects

{ $replaceWith: {
        $mergeObjects: [ "$$ROOT", "$subdoc" ]
} }