Join $graphLookup result with other collection - mongodb

Question

I'm working on a hierarchical structure that stores a binary tree.
Let's say I have two collections: users and nodes.
users collection stores personal information and nodes stores the structure of the tree using the Parent References pattern: https://docs.mongodb.com/manual/tutorial/model-tree-structures-with-parent-references/
Users:
[{
"_id": {
"$oid": "600365521599912a5c814e5e"
},
"nombre": "Andres",
"correo": "oachica#gmail.com"
},{
"_id": {
"$oid": "600365e9ccf1e51b2cab341f"
},
"nombre": "Andres",
"correo": "cachi777_#hotmail.com"
},{
"_id": {
"$oid": "6004591536a40941f48121f9"
},
"nombre": "Laura",
"correo": "w.l777#hotmail.com"
},{
"_id": {
"$oid": "6004596936a40941f48121fb"
},
"nombre": "Javi",
"correo": "jocta#hotmail.com"
},{
"_id": {
"$oid": "60047cf23f3f1a0d647cb2c7"
},
"nombre": "Lina",
"correo": "lvelas#hotmail.com"
}]
nodos:
[{
"_id": {
"$oid": "60035d0a1599912a5c814e58"
},
"idUsuario": "600365521599912a5c814e5e",
"nodoPadre": ""
},{
"_id": {
"$oid": "60047e6874cab54a7088ca56"
},
"idUsuario": "600365e9ccf1e51b2cab341f",
"nodoPadre": {
"$oid": "60035d0a1599912a5c814e58"
}
},{
"_id": {
"$oid": "60047f42c89add3c20cff990"
},
"idUsuario": "6004591536a40941f48121f9",
"nodoPadre": {
"$oid": "60047e6874cab54a7088ca56"
}
},{
"_id": {
"$oid": "60047f5dc89add3c20cff991"
},
"idUsuario": "6004596936a40941f48121fb",
"nodoPadre": {
"$oid": "60047f42c89add3c20cff990"
}
},{
"_id": {
"$oid": "600480de9fd6a42b40679e6d"
},
"idUsuario": "60047cf23f3f1a0d647cb2c7",
"nodoPadre": {
"$oid": "60047f5dc89add3c20cff991"
}
}]
Each document in nodos has corresponding document in users bound by _id and idUsuario.
Generally a document in nodes collection has a parent node in the same collection bound by nodoPadre field.
I'm able to get childs of a node using $graphLookup aggregation:
As you can se I got the childs of a node. Now I need to put the personal information in each child in the result array "hijos" as shown below:
Thanks for your help.

Answer 1

$graphLookup as per your requirement
$unwind deconstruct hijos array
$addFields convert hijos.idUsuario to object id because its an string, if it is already in object id then remove this stage
$lookup with users collection
$unwind deconstruct hijos.idUsuario array
$addFields to remove hijos if it is blank {} object
$group by _id and reconstruct hijos array
db.nodes.aggregate([
{
"$graphLookup": {
"from": "nodes",
"startWith": "$_id",
"connectFromField": "_id",
"connectToField": "nodoPadre",
"as": "hijos",
"maxDepth": 4
}
},
{
$unwind: {
path: "$hijos",
preserveNullAndEmptyArrays: true
}
},
{ $addFields: { "hijos.idUsuario": { $toObjectId: "$hijos.idUsuario" } } },
{
"$lookup": {
"from": "users",
"localField": "hijos.idUsuario",
"foreignField": "_id",
"as": "hijos.idUsuario"
}
},
{
$unwind: {
path: "$hijos.idUsuario",
preserveNullAndEmptyArrays: true
}
},
{
$addFields: {
hijos: {
$cond: [{ $eq: ["$hijos", {}] }, "$$REMOVE", "$hijos"]
}
}
},
{
$group: {
_id: "$_id",
hijos: { $push: "$hijos" },
idUsuario: { $first: "$idUsuario" },
nodoPadre: { $first: "$nodoPadre" }
}
}
])
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