MongoDB Aggregation - How to condition for $set - node.js

Question

I am unwinding a lookup joined by { preserveNullAndEmptyArrays: true } option to include documents whose sizes field is null, missing, or an empty array. But in my case, I need to join nested lookup and the output result is not as I expected.
Specifically, I am being to set a new buyer field by the $set operator from lookup but if this lookup is null or empty array it will return [ { } ] and I don't want that. How can I replace [ { } ] with empty array?
The result I got:
[
{
"_id": ObjectId("5a934e000102030405000000"),
"buyers": [
{
"_id": ObjectId("5a934e000102030405000003"),
"buyer": {
"_id": ObjectId("5a934e000102030405000004"),
"display_name": "Phineas",
"user_id": "123"
},
"post_id": 1,
"user_id": "123"
}
],
"content": "content book 1",
"title": "title book 1"
},
{
"_id": ObjectId("5a934e000102030405000002"),
"buyers": [
{}
],
"content": "content book 3",
"title": "title book 3"
},
{
"_id": ObjectId("5a934e000102030405000001"),
"buyers": [
{}
],
"content": "content book 2",
"title": "title book 2"
}
]
The result that I expected:
[
{
"_id": ObjectId("5a934e000102030405000000"),
"buyers": [
{
"_id": ObjectId("5a934e000102030405000003"),
"buyer": {
"_id": ObjectId("5a934e000102030405000004"),
"display_name": "Phineas",
"user_id": "123"
},
"post_id": 1,
"user_id": "123"
}
],
"content": "content book 1",
"title": "title book 1"
},
{
"_id": ObjectId("5a934e000102030405000002"),
"buyers": [],
"content": "content book 3",
"title": "title book 3"
},
{
"_id": ObjectId("5a934e000102030405000001"),
"buyers": [],
"content": "content book 2",
"title": "title book 2"
}
]
Now you can see my code in the Mongo playground link:
https://mongoplayground.net/p/qiC7FySejxF

Answer 1

Comparing your actual output and MongoDB Playground link, I believe that you miss out the $unset stage.
{
$unset: "buyers.buyers"
}
While after the $unset stage, you can add $set stage to update the buyers field by filtering the document which is not empty document via $filter.
{
$set: {
buyers: {
$filter: {
input: "$buyers",
cond: {
$ne: [
"$$this",
{}
]
}
}
}
}
}
Demo # Mongo Playground