How to group same record into multiple groups using mongodb aggregate pipeline - mongodb

Question

I have a two collections.
OrgStructure (visualise this as a tree structure)
Example Document:
{
"id": "org1",
"nodes": [
{
"nodeId": "root",
"childNodes": ["child1"]
},
{
"nodeId": "child1",
"childNodes": ["child2"]
},
{
"nodeId": "child2",
"childNodes": []
}
]
}
Activity
Example Document:
[
{
"id":"A1",
"orgUnit": "root"
},
{
"id":"A2",
"orgUnit": "child1"
},
{
"id":"A3",
"orgUnit": "child2"
}
]
Now my expectation is to group activities by orgUnit such a way that by considering the child nodes as well.
Here i don't want to do a lookup and i need to consider one OrgStructure document as an input, so that i can construct some condition using the document such a way that the query will return the below result.
Expected result
[
{
"_id": "root",
"activities": ["A1","A2","A3"]
},
{
"_id": "child1",
"activities": ["A2","A3"]
},
{
"_id": "child2",
"activities": ["A3"]
}
]
So im ecpecting an aggregate query something like this
{
"$group": {
"_id": {
"$switch": {
"branches": [
{
"case": {"$in": ["$orgUnit",["root","child1","child2"]]},
"then": "root"
},
{
"case": {"$in": ["$orgUnit",["child1","child2"]]},
"then": "child1"
},
{
"case": {"$in": ["$orgUnit",["child2"]]},
"then": "child2"
}
],
"default": null
}
}
}
}
Thanks in advance!

Answer 1

You will need 2 steps:
create another collection nodes for recursive lookup. The original OrgStructure is hard to perform $graphLookup
db.OrgStructure.aggregate([
{
"$unwind": "$nodes"
},
{
"$replaceRoot": {
"newRoot": "$nodes"
}
},
{
$out: "nodes"
}
])
Perform $graphLookup on nodes collection to get all child nodes. Perform $lookup to Activity and do some wrangling.
db.nodes.aggregate([
{
"$graphLookup": {
"from": "nodes",
"startWith": "$nodeId",
"connectFromField": "childNodes",
"connectToField": "nodeId",
"as": "nodesLookup"
}
},
{
"$lookup": {
"from": "Activity",
"let": {
nodeId: "$nodesLookup.nodeId"
},
"pipeline": [
{
$match: {
$expr: {
$in: [
"$orgUnit",
"$$nodeId"
]
}
}
},
{
$group: {
_id: "$id"
}
}
],
"as": "activity"
}
},
{
$project: {
_id: "$nodeId",
activities: "$activity._id"
}
}
])
Here is the Mongo playground for your reference.