mongodb - summations of array length with same ids - mongodb

Question

I am creating a platform where people can share their memes. On one page I want to show them who are the most popular members on the platform. so, there is a collection of 'meme' and 'user'
for example,
There is two content with same ids:
{
_id: 1,
username: "name",
bio: "bio",
image: "url",
};
memes
{
_id: 0,
user_id: 1,
image: "meme1.jpg",
likes: [
{
user_id: 4
}
]
},
{
_id: 1,
user_id: 1,
image: "meme2.jpg",
likes: [
{
user_id: 5
},
{
user_id: 6
}
]
}
and I want to output something like this way
{
user_id:1,
username:"name"
likes:3,
}
I wrote this query using aggregate functions but I am not understanding how to identify ids are the same or not?
meme
.aggregate([
{
$lookup: {
from: "users",
localField: "user_id",
foreignField: "_id",
as: "userDetails",
},
},
{
$project: {
user_id: "$user_id",
username: "$userDetails.username",
likes: {
$size: "$likes",
},
},
},
{
$sort: { likes: 1 },
},
])
.exec()
.then((result) => {
console.log(result);
});

Answer 1

It will be easier to start query with users.
You can use $sum, $map, $size aggregations to get the total likes, and add it using $addFields.
db.users.aggregate([
{
$lookup: {
from: "memes",
localField: "_id",
foreignField: "user_id",
as: "userDetails"
}
},
{
$addFields: {
"likes": {
"$sum": {
"$map": {
"input": "$userDetails",
"in": {
"$size": "$$this.likes"
}
}
}
}
}
},
{
$project: {
_id: 0,
user_id: "$_id",
username: 1,
likes: 1
}
}
])
Playground
Result:
[
{
"likes": 3,
"user_id": 1,
"username": "name"
}
]

Answer 2

You could project the length of the likes-array and group each projection by the user_id and cound the results. Something like this should work:
db.getCollection('memes').aggregate([{
$lookup: {
from: "users",
localField: "user_id",
foreignField: "_id",
as: "userDetails"
}
}, {
"$project": {
"user_id": 1,
"likesSize": {
"$size": "$likes"
}
}
}, {
$group: {
_id: "$user_id",
"count": {
"$sum": "$likesSize"
}
}
}
])
The above query should return:
{
"_id" : 1,
"count" : 3
}