Mongodb: compute a value before using it as connectFromField in an aggregate - mongodb

Question

I have mongo tree structure that looks like this:
{"_id":uid1,"parent": null, "path": "#uid1", "name": "a"}
{"_id":uid2,"parent": "uid1", "path": "#uid1#uid2", "name": "b"}
{"_id":uid3,"parent": "uid1", "path": "#uid1#uid3", "name": "c"}
{"_id":uid4,"parent": "uid2", "path": "#uid1#uid2#uid4", "name": "1"}
{"_id":uid5,"parent": "uid2", "path": "#uid1#uid2#uid5", "name": "2"}
{"_id":uid6,"parent": "uid1", "path": "#uid1#uid6", "name": "1"}
{"_id":uid7,"parent": "uid6", "path": "#uid1#uid6#uid7", "name": "x"}
where every node is represented by its unique id uidx and located thanks to its parent's uid. Everytime the parent of a node is modified its path and the paths of its children are automatically modified (inside a mongoose pre-save).
The example above can be represented as follows:
a
|_b
| |_1
| |_2
|_c
|_1
|_x
My goal is to build a request that will get only the leaves under a specified node.
Had I stored the parent path inside the field parent instead of only the parent identifier I would have been able to do it using the following request:
db.tree.aggregate([
{$match:{"parent": {$regex:"^#uid1#uid2"}}},
{$graphLookup:{
from:"tree",
startWith:"$path",
connectFromField:"path",
connectToField:"parent",
as:"dep"}},
{$match:{dep:[]}},
{$project:{"_id":0, path:1}}
])
as already answered in my previous question here: Mongodb: get only leaves of tree
The problem is I did not.
So I have to somehow transform the 'connectToField' in my request so that it represents the path of my parent instead of the id of my parent. Does anybody have an idea on how to do this?
This question is a rewritten version of my previous question here:previous version

Answer 1

You don't need to calculate anything, nor rely on the path.
It is the exact usecase from https://docs.mongodb.com/manual/reference/operator/aggregation/graphLookup/. The { $match: { dep: [] } } stage returns nodes with no children, i.e. leaves.
db.tree.aggregate([
{ $graphLookup: {
from:"tree",
startWith:"$_id",
connectFromField:"_id",
connectToField:"parent",
as:"dep",
maxDepth: 1
} },
{ $match: { dep: [] } },
])
maxDepth: 1 is added to speed things up a bit. As soon as the node have a direct child you don't care about rest of the branch, so 1 level depth for direct children is sufficient: