Remove documents not linked by DBRef mongodb - mongodb

Question

Hello everyone I try to remove all documents which are no link by an DBRef field.
For example I have 2 collections User and Order.
Let's say User collection has this structure :
{
"_id": "12345",
"lastName": "Michael",
"firstName": "Bernard"
}
And Order collection has this structure :
{
"_id": "123456",
"orderDate": "2022-12-26",
"userWhoOrder": {
"$ref": "user",
"$id": "12345"
},
"userWhoPay": {
"$ref": "user",
"$id": "123456"
}
}
Note : userWhoOrder and userWhoPay are DBRef fields (and it can be the same user).
So I want remove all users who are not present in order collection (neither userWhoOrder neither userWhoPay).
I know I can do this in 2 steps :
Get a list of userWhoOrder and userWhoPay from order collection.
Filter users list which not contains users of step 1 and remove them.
But I want to know if there are a properly way to do this with a single request (using $lookup for example).
Here is what I tried for getting a users list to remove :
db.getCollection("user").aggregate[{$lookup: {
from: "order",
let: {userId: "$_id"},
pipeline: [
{$match: {$expr: {
$and: [
{$ne: ["$userWhoOrder.$id", "$$userId"]},
{$ne: ["$userWhoPay.$id", "$$userId"]}
]
}}}
],
as: "result"
}]