Move embedded documents in mongoDB into new documents

Question

{
  "_id": {
    "$oid": "2534c6893d643531411"
  },
  "id": "854",
  "920543": {
    "createdAt": 1667210991769,
  },
  "750544": {
    "createdAt": 1693292609,
  },
  "719572": {
    "createdAt": 1693292739,
}

I am trying to go through every document in a collection and move all the embedded documents within the document and create a new document for each embedded document with the key such 750544, 719572 as the messageId field in the new id, and the rest of the data such as createdAt as a normal field. I also want to keep the id field the same in the new documents.

Example documents based on the above would look like

{
  "_id": {
    "$oid": "someIdGeneratedbyMongo"
  },
  "id": "854",
  "messageId": "920543",
  "createdAt": 1667210991769
},
{
  "_id": {
    "$oid": "someIdGeneratedbyMongo"
  },
  "id": "854",
  "messageId": "750544",
  "createdAt": 1693292609
},
{
  "_id": {
    "$oid": "someIdGeneratedbyMongo"
  },
  "id": "854",
  "messageId": "719572",
  "createdAt": 1693292739
}

etc

Answer 1

1. $set - Create a data array field with the value of converting all the fields in the current document (excluding the _id and id fields) to the document with k and v fields.

2. $unwind - Deconstruct the data array field.

3. $project - Decorate the output documents.

db.collection.aggregate([
  {
    $set: {
      data: {
        $filter: {
          input: {
            $objectToArray: "$$ROOT"
          },
          cond: {
            $not: {
              $in: [
                "$$this.k",
                [
                  "_id",
                  "id"
                ]
              ]
            }
          }
        }
      }
    }
  },
  {
    $unwind: "$data"
  },
  {
    $project: {
      id: 1,
      messageId: "$data.k",
      createdAt: "$data.v.createdAt"
    }
  }
])

Demo @ Mongo Playground