aggregation lookup and match a nested array - mongodb

Question

Hello i am trying to join two collections...
#COLLECTION 1
const valuesSchema= new Schema({
value: { type: String },
})
const categoriesSchema = new Schema({
name: { type: String },
values: [valuesSchema]
})
mongoose.model('categories', categoriesSchema )
#COLLECTION 2
const productsSchema = new Schema({
name: { type: String },
description: { type: String },
categories: [{
type: mongoose.Schema.Types.ObjectId,
ref: 'categories',
}]
})
mongoose.model('productos', productsSchema )
Now, what i pretend to do is join these collections and have an output like this.
#Example Product Document
{
name: 'My laptop',
description: 'Very ugly laptop',
categories: ['5f55949054f3f31db0491b5c','5f55949054f3f31db0491b5b'] // these are _id of valuesSchema
}
#Expected Output
{
name: 'My laptop',
description: 'Very ugly laptop',
categories: [{value: 'Laptop'}, {value: 'PC'}]
}
This is what i tried.
{
$lookup: {
from: "categories",
let: { "categories": "$categories" },
as: "categories",
pipeline: [
{
$match: {
$expr: {
$in: [ '$values._id','$$categories']
},
}
},
]
}
}
but this query is not matching... Any help please?

Answer 1

You can try,
$lookup with categories
$unwind deconstruct values array
$match categories id with value id
$project to show required field
db.products.aggregate([
{
$lookup: {
from: "categories",
let: { cat: "$categories" },
as: "categories",
pipeline: [
{ $unwind: "$values" },
{ $match: { $expr: { $in: ["$values._id", "$$cat"] } } },
{
$project: {
_id: 0,
value: "$values.value"
}
}
]
}
}
])
Playground

Answer 2

Since you try to use the non-co-related queries, I appreciate it, you can easily achieve with $unwind to flat the array and then $match. To regroup the array we use $group. The $reduce helps to move on each arrays and store some particular values.
[
{
$lookup: {
from: "categories",
let: {
"categories": "$categories"
},
as: "categories",
pipeline: [
{
$unwind: "$values"
},
{
$match: {
$expr: {
$in: [
"$values._id",
"$$categories"
]
},
}
},
{
$group: {
_id: "$_id",
values: {
$addToSet: "$values"
}
}
}
]
}
},
{
$project: {
categories: {
$reduce: {
input: "$categories",
initialValue: [],
in: {
$concatArrays: [
"$$this.values",
"$$value"
]
}
}
}
}
}
]
Working Mongo template