How to get all children documents using $graphLookup of MongoDB

Question

I have this data as below in two collections of MongoDB. categories:

[
  { "_id" : 1, "name" : "A" },
  { "_id" : 2, "name" : "B", "categoryId" : 1 },
  { "_id" : 3, "name" : "C", "categoryId" : 1 },
  { "_id" : 4, "name" : "D", "categoryId" : 2 },
  { "_id" : 5, "name" : "E", "categoryId" : 3 },
  { "_id" : 6, "name" : "F", "categoryId" : 2 }
];

And I have Locos documents:

[
  { "_id" : 1, "name" : "X", "categoryId" : 2 },
  { "_id" : 2, "name" : "Y", "categoryId" : 3 },
  { "_id" : 2, "name" : "B", "categoryId" : 1 } 
]

For example if I want to get all the chilren of category A which has the id 1, i call the function and it returns an array of ids of children categories and if possible the Locos chilren. So the result will be like this:

chilren: {
          categories: [2, 3, 4, 5, 6],
          locos: [1, 2, 3]
         }

If i call the function with the id 2 which is category B, I get the result:

chilren: {
          categories: [2, 6],
          locos: [1]
         }

Answer 1

You are on the right track to use $graphLookup.

db.categories.aggregate([
  {
    "$match": {
      _id: 1
    }
  },
  {
    "$graphLookup": {
      "from": "categories",
      "startWith": "$_id",
      "connectFromField": "_id",
      "connectToField": "categoryId",
      "as": "categoryLookup"
    }
  },
  {
    "$graphLookup": {
      "from": "locos",
      "startWith": "$_id",
      "connectFromField": "_id",
      "connectToField": "categoryId",
      "as": "locosLookup"
    }
  },
  {
    "$project": {
      children: {
        categories: {
          "$map": {
            "input": "$categoryLookup",
            "as": "c",
            "in": "$$c._id"
          }
        },
        locos: {
          "$map": {
            "input": "$locosLookup",
            "as": "l",
            "in": "$$l._id"
          }
        }
      }
    }
  }
])

Here is the Mongo playground for your reference.