How to get linked data from 2 collections with aggregation mongoDB - node.js

Question

I'm new as MongoDB, I have 2 collections: User and Video with below structure. When user like a video, video's _id will be added to liked_videos on user collection. liked_videos is an array of _id
video collection
{
_id
mp4_url,
liked_count
}
User collection
{
_id,
username,
password,
liked_videos: [ // videos _id array ]
}
How do I query to get the user's liked videos? Like below?
[
{
_id: 1
mp4_url,
liked_count
},
{
_id: 2
mp4_url,
liked_count
},
...
]
Thank you

Answer 1

You need to use $lookup to join collections and $size to get the array size
db.video.aggregate([
{
"$lookup": {
"from": "user",
"localField": "_id",
"foreignField": "liked_videos",
"as": "join_video"
}
},
{
"$project": {
liked_count: {
$size: "$join_video"
}
}
}
])
Working Mongo playground

Answer 2

Try this way
db={
"video": [
{
"_id": 1,
"mp4_url": "url1",
"liked_count": 2,
"quantity": 2
},
{
"_id": 2,
"mp4_url": "url2",
"liked_count": 3,
"quantity": 1
},
{
"_id": 3,
"mp4_url": "url2",
"liked_count": 3,
"quantity": 4
}
],
"User": [
{
"_id": 1,
"username": "almonds",
"password": "pwd",
"Likedvideos": [
"aaa",
"ffff"
]
},
{
"_id": 2,
"username": "almonds",
"password": "pwd",
"Likedvideos": [
"qqq",
"bbbb"
]
},
{
"_id": 3,
"username": "almonds",
"password": "pwd",
"Likedvideos": [
"ccc",
"ffff"
]
},
{
"_id": 4,
"username": "almonds",
"password": "pwd",
"Likedvideos": [
"bbbb",
"ffff"
]
},
{
"_id": 5,
"username": "almonds",
"password": "pwd",
"Likedvideos": [
"qqq",
"ffff"
]
},
{
"_id": 6
}
]
}
Query :
db.video.aggregate([
{
$match: {
_id: 1
}
},
{
"$lookup": {
"from": "User",
"localField": "_id",
"foreignField": "_id",
"as": "data"
},
},
{
$unwind: "$data"
},
{
$replaceRoot: {
newRoot: {
$mergeObjects: [
"$data",
"$$ROOT"
]
}
}
},
{
$project: {
data: 0
}
}
])
Output:
[
{
"Likedvideos": [
"aaa",
"ffff"
],
"_id": 1,
"liked_count": 2,
"mp4_url": "url1",
"password": "pwd",
"quantity": 2,
"username": "almonds"
}
]
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