mongodb lookup with foreign field match - mongodb

Question

I want to know how can i lookup with match.
I could lookup two collection by this query.
db.category.aggregate([
{
$lookup:
{
from: "faq",
localField: "_id",
foreignField: "category_code",
as: "faq"
}
}
])
Then i get this result.
{
"_id":ObjectId("1234"),
"category_name":"about account",
"faq": [
{
"_id":ObjectId("faq id blah blah"),
"category_code" : ObjectId("1234"),
"faq_title":"When you can't create account",
"del_flg":"N"
},
{
"_id":ObjectId("faq id blah blah2222"),
"category_code" : ObjectId("1234"),
"faq_title":"When you change your account",
"del_flg":"N"
},
{
"_id":ObjectId("faq id blah blah3333"),
"category_code" : ObjectId("1234"),
"faq_title":"When you lost your account",
"del_flg":"Y"
}
[
}
I want just two faq item which "del_flg" is "N"
I am not familiar to use mongodb.
Is it possible if i use pipline? I tried some pipline examples, but i couldn't.

Answer 1

You should try $lookup with pipeline,
let will create a variable to access id inside lookup pipeline using $$
pipeline match using $expr because we are comparing both fields $$id = $category_code
set match condition del_flg: N
db.category.aggregate([
{
$lookup: {
from: "faq",
let: {
id: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$$id",
"$category_code"
]
},
"del_flg": "N"
}
}
],
as: "faq"
}
}
])
Playground: https://mongoplayground.net/p/gUVUMk6KIAV