Select data from two MongoDB tables and update the results - mongodb

Question

We have the following problem
Given are the tables and fields
Offer
OfferId
State
Article
OfferId
ArticleId
NetPrice
GrossPrice
VatRate
Example-data:
Offer-Collection
{
"_id": "1",
"State": "INITIAL",
"_class": "com.example.dto.OfferData"
}
{
"_id": "2",
"State": "COMPLETED",
"_class": "com.example.dto.OfferData"
}
Article-Collection
{
"_id": {
"$oid": "a"
},
"Description": "asdf",
"NetPrice": "100",
"GrossPrice": "116",
"VatRate": "16",
"OfferId": "1",
"_class": "com.example.dto.Article"
}
{
"_id": {
"$oid": "b"
},
"Description": "my description",
"NetPrice": "100",
"GrossPrice": "119",
"VatRate": "19",
"OfferId": "1",
"_class": "com.example.dto.Article"
}
{
"_id": {
"$oid": "c"
},
"Description": "my description",
"NetPrice": "100",
"GrossPrice": "116",
"VatRate": "16",
"OfferId": "2",
"_class": "com.example.dto.Article"
}
Now we have to update all articles belonging to an offer with the state "initial" in the following way: if the VatRate is equal to 16 than it must be updated to 19 AND the GrossPrice must be recalculated from the existing NetPrice.
The result should be: the article with _id = "a" and VatRate = 16 for OfferId = 1 (State = INITIAL) should have VatRate = 19 and GrossPrice = 119. The fields should be updated and persisted in the original MongoDB collection.
Can we do this only with Mongo-shell? Our Version is 3.6.
Our tries:
We have played around with .aggregate, $lookup, $match and $project but without much luck. It's the first time we are using the Mongo-shell.
db.getCollection("Offers").aggregate([{
$lookup:{
from:"Articles",
localField:"OfferId",
foreignField:"OfferId",
as:"selected-articles"
}
},
{
$match: { "state": { "$eq": "INITIAL" } }
},
{
$project: { "articles": 1 }
}
]).forEach(...?)

Answer 1

$match your State condition
$lookup with Articles collection
$map to iterate loop of selected-articles array, check condition using $cond if VatRate is "16" then updated to 19 and recalculate GrossPrice as per NetPrice using $multiply before it convert NetPrice to integer because its in string type, back to merge objects with current objects using $mergeObjects
db.getCollection("Offers").aggregate([
{ $match: { State: { $eq: "INITIAL" } } },
{
$lookup: {
from: "Articles",
localField: "_id",
foreignField: "OfferId",
as: "selected-articles"
}
},
{
$addFields: {
"selected-articles": {
$map: {
input: "$selected-articles",
in: {
$mergeObjects: [
"$$this",
{
$cond: [
{ $eq: ["$$this.VatRate", "16"] },
{
VatRate: 19,
GrossPrice: {
$multiply: [{ $toInt: "$$this.NetPrice" }, 19]
}
},
{}
]
}
]
}
}
}
}
}
])
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