MongoDB - Lookup with pipeline get error: $nin needs an array - mongodb

Question

Hello everyone I am having some trouble basically I am getting
$nin needs an array
even after passing an array but using a variable when attempting to run the following query.
I want to get documents from a collection numberData that contains a property value which is not assigned to an array in another collection named assignedData.
Example of numbersData docs are given below:
/* 1 */
{
"_id" : ObjectId("61f2b5923327b3f6f4dad27e"),
"value" : 1.0
}
/* 2 */
{
"_id" : ObjectId("61f2b5923327b3f6f4dad27f"),
"value" : 2.0
}
/* 3 */
{
"_id" : ObjectId("61f2b5923327b3f6f4dad280"),
"value" : 3.0
}
/* 4 */
{
"_id" : ObjectId("61f2b5923327b3f6f4dad281"),
"value" : 4.0
}
and the contents of assignedData are given below:
/* 1 */
{
"_id" : ObjectId("61f2b5e43327b3f6f4dad282"),
"name" : "john",
"assigned" : [
1.0,
4.0
]
}
The query I used:
db.assignedData.aggregate([
{
//i only want to get single doc from assigned data
//so using this match
$match: {
_id: ObjectId("61f2b5e43327b3f6f4dad282")
},
},
{
$lookup: {
from: 'numbersData',
let: { aArr: "$assigned" },
pipeline: [
{
$match: {
//here i am using $$maArr
//which is refering to assigned array
value:
{
$nin: "$$aArr"
}
}
}
],
as: 'matchData'
}
}
])
After running this query I got the error:
$nin needs an array
I don't know why please suggest me solution if you can.

Answer 1

From Join Conditions and Subqueries on a Joined Collection section (refer to the table),
let
A $match stage requires the use of an $expr operator to access the variables. The $expr operator allows the use of aggregation expressions inside of the $match syntax.
Instead of using $nin operator, change as below:
{
$match: {
$expr: {
$not: {
$in: [
"$value",
"$$aArr"
]
}
}
}
}
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