get sum conditional with mongodb - arrays

Question

I need to get the sells sum, just with the idCategory, any idea?
I have this 3 schemas in mongondb
category = [{"id":1,"name":"cat1"}, {"id":2,"name":"cat2"}]
product = [{"id":1,"name":"product1", "catId":1}, {"id":2,"name":"product2", "catId":2}]
sells = [{"id":1,"value":80, "productId":1, status:'active'}, {"id":2,"value":90, "productId":2, status:'Inactive'}]

Answer 1

MongoDB collections are not actual schemas, also, I'm assuming:
You're interpreting them as arrays (although they aren't), so you mean each of your Collections have 2 documents inside of them.
You want the sum of sales grouped by product category.
If those are the cases, what you want is a MongoDB Aggregate which you would run on your "sells" collection, "join" with the "product" collection and group by category id.
The base aggregate to do so would be along the following lines:
sells.aggregate([
{
$lookup: {
from: "product",
localField: "productId",
foreignField: "id",
as: "ProductData"
}
},
{
$unwind: "$ProductData"
},
{
$group: {
_id: "$ProductData.catId",
total: { $sum: "$value" }
}
}
]);
If you also want to fetch the Category name after aggregating, all you need to do is insert another $lookup at the end of the pipeline joining with category collection:
sells.aggregate([
{
$lookup: {
from: "product",
localField: "productId",
foreignField: "id",
as: "ProductData"
}
},
{
$unwind: "$ProductData"
},
{
$group: {
_id: "$ProductData.catId",
total: { $sum: "$value" }
}
},
{
$lookup: {
from: "category",
localField: "_id",
foreignField: "id",
as: "CategoryData"
}
},
{
$unwind: "$CategoryData"
},
{
$project: {
name: "$CategoryData.name",
total: 1
}
}
]);
EDIT (adding new case request on comment):
db.getCollection('product').aggregate([
{
$lookup: {
from: "sells",
localField: "id",
foreignField: "productId",
as: "SalesData"
}
},
{
$unwind:
{
path: "$SalesData",
preserveNullAndEmptyArrays: true
}
},
{
$project: {
catId: 1,
value: "$SalesData.value"
}
},
{
$group: {
_id: "$catId",
total: { $sum: "$value" }
}
},
{
$lookup: {
from: "category",
localField: "_id",
foreignField: "id",
as: "CategoryData"
}
},
{
$unwind: "$CategoryData"
},
{
$project: {
name: "$CategoryData.name",
total: 1
}
}
]);