How to find by id after lookup aggregation - mongodb

Question

I have a collection of news articles in mongodb and another collection that maps a user's ID to an article's ID and has a "like" state, which can be either "like" or "dislike" or "none" if no entry with the user and article exists.
Here are both schemas:
// news collection
const articleSchema = new Schema({
title: String,
content: String,
})
// newslikes collection
const articleLikeSchema = new Schema({
user: { type: Schema.Types.ObjectId, ref: 'Client' },
article: { type: Schema.Types.ObjectId, ref: 'News' },
state: { type: String, enum: ['like', 'dislike'] }
})
I'm trying to write an aggregation query which joins these two collection using $lookup and then finds the state of a specific user's like on all articles. This is what I have so far:
const results = await News.aggregate([
{ $match: query },
{ $sort: { date: -1 } },
{ $skip: page * pageLength },
{ $limit: pageLength },
{ $lookup: {
from: 'newslikes',
localField: '_id',
foreignField: 'article',
as: 'likes'
} },
{ $project: {
title: 1,
likes: 1,
content: 1,
// numLikes: { $size: '$likes' }
userLikeStatus: {
$filter: {
input: '$likes',
as: 'like',
cond: {
$eq: ['$user._id', '5ccf13adcec5e6d84f940417']
}
}
}
} }
])
However this is not working. Is what I'm doing even the correct approach or is there a better way to do this rather than $filter?

Answer 1

You can use below aggregation with mongodb 3.6 and above
News.aggregate([
{ "$match": query },
{ "$sort": { "date": -1 } },
{ "$skip": page * pageLength },
{ "$limit": pageLength },
{ "$lookup": {
"from": "newslikes",
"let": { "articleId": "$_id" },
"pipeline": [
{ "$match": {
"$expr": { "$eq": [ "$article", "$$articleId" ] },
"user": mongoose.Types.ObjectId("5ccf13adcec5e6d84f940417")
}}
],
"as": "likes"
}},
{ "$addFields": {
"userLikeStatus": { "$ifNull": [{ "$arrayElemAt": ["$likes.state", 0] }, "none"] }
}}
])
Or the way you are trying
Basically here you need to put $cond for the field userLikeStatus i.e if the $size of the array after $filter is $gte 1 then user likes it else does not.
News.aggregate([
{ "$match": query },
{ "$sort": { "date": -1 } },
{ "$skip": page * pageLength },
{ "$limit": pageLength },
{ "$lookup": {
"from": "newslikes",
"localField": "_id",
"foreignField": "article",
"as": "likes"
}},
{ "$project": {
"title": 1,
"likes": 1,
"content": 1,
// numLikes: { $size: '$likes' }
"userLikeStatus": {
"$let": {
"vars": {
"array": {
"$filter": {
"input": "$likes",
"as": "like",
"cond": { "$eq": ["$$like.user", mongoose.Types.ObjectId("5ccf13adcec5e6d84f940417")] }
}
}
},
"in": {
"$ifNull": [{ "$arrayElemAt": ["$$array.state", 0] }, "none"]
}
}
}
}}
])