Cannot get parent data by mongoDB aggregate graphLookup - mongodb

Question

Following is the data
[
{
"_id": {
"$oid": "6364f2eee35fc06fa06afb5f"
},
"type": "subbranch",
"parent_id": "635116c18fe4294398842ebb",
"org_name": "Pune - Delhi"
},
{
"_id": {
"$oid": "635116c18fe4294398842ebb"
},
"type": "branch",
"org_name": "Delhi Branch",
"parent_id": "0"
}
]
query which i have written is as follows
// req.params.id is 6364f2eee35fc06fa06afb5f
let id = mongoose.Types.ObjectId(req.params.id);
let data = await organisation.aggregate([
{
$addFields: { "_idString": { "$toString": "$_id" }}
},
{
$graphLookup: {
from: "organisations",
startWith: "$parent_id",
connectFromField: "parent_id",
connectToField: "_idString",
as: "parent"
}
},
{
$match: {_id: id}
},
]);
but in output i get as follow
[
{
"_id": "6364f2eee35fc06fa06afb5f",
"type": "subbranch",
"parent_id": "635116c18fe4294398842ebb",
"org_name": "Pune - Delhi",
"_idString": "6364f2eee35fc06fa06afb5f",
"parent": [
]
}
]
i am getting empty parent array but expected output is array with parent data in it.
any suggestion would be appreciated.

Answer 1

Remember connectFromField expected or extracted from current aggregated collection while connectToField is connected to from orginal collection
DEMO ON https://mongoplayground.net/p/vYDdOgNt9bW
The aggregate query be like
db.collection.aggregate([
{
$addFields: {
"parent_id": {
$convert: {
input: "$parent_id",
to: "objectId",
onError: "$parent_id",
}
}
}
},
{
$graphLookup: {
from: "collection",
startWith: "$parent_id",
connectFromField: "parent_id",
connectToField: "_id",
as: "parent"
}
}
])
Outputs
[
{
"_id": ObjectId("6364f2eee35fc06fa06afb5f"),
"org_name": "Pune - Delhi",
"parent": [
{
"_id": ObjectId("635116c18fe4294398842ebb"),
"org_name": "Delhi Branch",
"parent_id": "0",
"type": "branch"
}
],
"parent_id": ObjectId("635116c18fe4294398842ebb"),
"type": "subbranch"
},
{
"_id": ObjectId("635116c18fe4294398842ebb"),
"org_name": "Delhi Branch",
"parent": [],
"parent_id": "0",
"type": "branch"
}
]