mongodb push values into array with $reduce - mongodb

Question

I have an document structure like this:
_id: 123,
posts: [
{
_id: 234,
likesUser: [345, 456, 567, 678]
}
]
I want to use $reduce here so my expected output should look like this:
output: {
likes: [23, 31, 12, 32], //each element is the size of the "likesUser" array
ids: ["14312", "2342", "2312", "235234"] //each element is the "_id" in the posts array
}
I have try it like this:
let result = await Post.aggregate([
{
$match: {
_id: USER
}
},
{
$project: {
posts: {
$reduce: {
input: "$posts",
initialValue: { likes: [], ids: [] },
in: {
likes: {
$push: { $size: "$$this.likesUser" }
},
ids: {
$push: "$$this._id"
}
}
}
}
}
}
]);
In my frontend i use chart.js. The yAxsis are the likes and the xAxsis are the ID's. So for every element in my posts array i need the length of likesUser and _id
But it doenst work. Has anybody an idea how to achive this?

Answer 1

You are very close, you just have a minor syntax error in your $reduce, Try this:
let result = await Post.aggregate([
{
$match: {
_id: USER
}
},
{
$project: {
posts: {
$reduce: {
input: "$posts",
initialValue: {likes: [], ids: []},
in: {
likes: {
$concatArrays: ["$$value.likes", [{$size: "$$this.likesUser"}]]
},
ids: {
$concatArrays: ["$$value.ids", ["$$this._id"]]
}
}
}
}
}
}
]);

Answer 2

You are on the right way. You need to use $mergeObjects operator to achieve this. Following query will be helpful:
db.collection.aggregate([
{
$match: {
_id: USER
}
},
{
$project: {
post: {
$reduce: {
input: "$posts",
initialValue: {
likes: [],
ids: []
},
in: {
$mergeObjects: [
"$$value",
{
likes: {
$concatArrays: [
"$$value.likes",
[
{
$size: "$$this.likesUser"
}
]
]
},
ids: {
$concatArrays: [
"$$value.ids",
[
"$$this._id"
]
]
}
}
]
}
}
}
}
}
])
MongoPlayGroundLink
I hope the above is helpful.