How to query Child Reference Pattern collection with $graphLookup - mongodb

Question

I have a collection with child reference. Each document can have multiple parents.
How can I query it with $graphLookup in order to make a result prepared for a treeview?
Example:
{
"_id" : ObjectId("6143450cc0318c23d8f18424"),
"id" : "3",
"name" : "prod03",
"children" : [
{
"_id" : "6143440ac0318c23d8f1841f",
"qty" : 10
},
{
"_id" : "614344b1c0318c23d8f18422",
"qty" : 100
}
],
"totalQty" : 110
},
{
"_id" : ObjectId("614344b1c0318c23d8f18422"),
"id" : "2",
"name" : "prod02",
"children" : [ ],
"totalQty" : 100
},
{
"_id" : ObjectId("6143440ac0318c23d8f1841f"),
"id" : "1",
"name" : "prod01",
"children" : [ ],
"totalQty" : 10
}
Prod03 is formed from prod01 and prod02
The desired result would be like:
{
id: '3',
name: 'prod03',
totalQty: 110
children: [
{
id: '1',
name: 'prod01',
qty: 10
},
{
id: '2',
name: 'prod02',
qty: 100
},
],
}
The query must go multiple levels down until find no more children.
Final result would be a tree with all history of product manufacture components.

Answer 1

You are actually on the right track to use $graphLookup. You just need to convert children._id back to objectId from String for lookup.
db.collection.aggregate([
{
"$match": {
"id": "3"
}
},
{
"$addFields": {
"children": {
"$map": {
"input": "$children",
"as": "c",
"in": {
"_id": {
"$toObjectId": "$$c._id"
},
"qty": "$$c.qty"
}
}
}
}
},
{
"$graphLookup": {
"from": "collection",
"startWith": "$children._id",
"connectFromField": "children._id",
"connectToField": "_id",
"as": "children"
}
},
{
"$addFields": {
"children": {
"$map": {
"input": "$children",
"as": "c",
"in": {
"id": "$$c.id",
"name": "$$c.name",
"qty": "$$c.totalQty"
}
}
}
}
}
])
Here is the Mongo playground for your reference.