lookup with condition in mongoose - mongodb

Question

I have two collections. articles and bookmarks.
articles
{
_id: "5faa889ade5e0a6326a873d3",
name: "article 1"
},
{
_id: "5faa889ade5e0a6326a873d",
name: "article 2"
}
bookmarks
{
_id: "5faa889ade5e0a6326a873d1",
user_id: "5fc7b50da483a66a86aa7e9e",
model_id: "5faa889ade5e0a6326a873d3"
}
I want to join article with bookmark. if user bookmarked a article.
what i have tried
const aggregate = await Articles.aggregate([{
$lookup: {
from: "categories",
localField: "category_id",
foreignField: "_id",
as: "category_id"
}
},
{
$lookup: {
from: "bookmarks",
localField: "_id",
foreignField: "model_id",
as: "bookmarks"
}
}
]);
but it will gives all bookmark for the article not only logged in user bookmark. so how can I add a condition.
{ "user_id": objectId(req.user._id) } // logged user id

Answer 1

You can use $lookup with pipeline starting from MongoDB v3.6,
let to pass localField _id as model_id variable, you can use the field inside lookup pipeline using $$ reference,
pipeline to put $match stage and match your required conditions and user_id condition
{
$lookup: {
from: "bookmarks",
let: { model_id: "$_id" },
pipeline: [
{
$match: {
$expr: { $eq: ["$$model_id", "$model_id"] },
user_id: objectId(req.user._id)
}
}
],
as: "bookmarks"
}
}
Other option for MongoDB v3.4,
$filter to iterate loop of bookmarks and get filtered bookmarks on the base of condition
{
$lookup: {
from: "bookmarks",
localField: "_id",
foreignField: "model_id",
as: "bookmarks"
}
},
{
$addFields: {
bookmarks: {
$filter: {
input: "$bookmarks",
cond: { $eq: ["$$this.user_id", objectId(req.user._id)] }
}
}
}
}

Answer 2

You can have nested pipeline inside $lookup,
db.articles.aggregate([
{
$lookup: {
from: "bookmarks",
let: {
article_id: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$and: [
{
$eq: [
"$model_id",
"$$article_id"
]
},
{
$eq: [
"$user_id",
"5fc7b50da483a66a86aa7e9a"
]
}
]
}
}
}
],
as: "bookmarks"
}
}
])
Here's a working playground