Aggregating nested children ids by parentId in MongoDB

Question

There is parent-child structure like this:

folder-structure

The collection:

[
  {
    "_id": "1",
    "name": "document1",
    "parentId": null
  },
  {
    "_id": "2",
    "name": "folder1",
    "parentId": null,
    
  },
  {
    "_id": "3",
    "name": "folder2",
    "parentId": "2",
    
  },
  {
    "_id": "4"
    "name": "document2",
    "parentId": "2",
    
  },
  {
    "_id": "5"
    "name": "document3",
    "parentId": "3",
    
  }
]

The goal is to get folders with contentIds array in every folder, containing only direct children ids. The response should look like:

[
  {
    "_id": "2",
    "name": "folder1",
    "parentId": null,
    "contentIds": ["3", "4"]
    
  },
  {
    "_id": "3",
    "name": "folder2",
    "parentId": "2",
    "contentIds": ["5"]
    
  }
]

Answer 1

Just perform a self lookup and a $map to get your desired format of data.

db.collection.aggregate([
  {
    "$match": {
      name: {
        $regex: "folder"
      }
    }
  },
  {
    "$lookup": {
      "from": "collection",
      "localField": "_id",
      "foreignField": "parentId",
      "as": "contentIds"
    }
  },
  {
    "$addFields": {
      "contentIds": {
        "$map": {
          "input": "$contentIds",
          "in": "$$this._id"
        }
      }
    }
  }
])

Here is the Mongo playground for your reference.