Subquery with NOT IN with MongoDB - php

Question

I would like to create a query with mongodb which would do this thing:
SELECT * FROM Users WHERE Username NOT IN (SELECT LikeTo FROM Like WHERE Username = "John89").
i did something like this:
[
'$lookup'=> [
'from'=> "like",
'let'=> [ 'username'=> '$username' ],
'as'=> "leaders",
'pipeline'=> [
[
'$match'=> [
'$and'=> [
['$expr'=> [ '$not'=> ['$in'=> ['$likeTo','$$username']]]],
['username'=> "$username" ]
]
]
]
]
]
]
but i got PlanExecutor error during aggregation :: caused by :: $in requires an array as a second argument, found: string.
what did i do wrong? is the query right?
thanks!

Answer 1

I would personally split this into 2 queries, but here is how yo achieve this in a single pipeline, we actually want the lookup to match the "bad" users so we can use it to filter, like so:
db.users.aggregate([
{
"$lookup": {
"from": "likes",
let: {
username: "$username"
},
pipeline: [
{
$match: {
$expr: {
$and: [
{
$eq: [
"$likeTo",
"$$username"
]
},
{
"$eq": [
"$username",
"John89"
]
}
]
}
}
}
],
"as": "leaders"
}
},
{
$match: {
"leaders.0": {
$exists: false
}
}
},
{
$project: {
leaders: 0
}
}
])
Mongo Playground