MongoDB $lookup with conditional foreignField - mongodb

Question

Playground: https://mongoplayground.net/p/OxMnsCFZpmQ
My MongoDB version: 4.2.
I have a collection car_parts and customers.
As the name suggests car_parts has car parts, where some of them can have a field sub_parts which is a list of car_parts._ids this part consists of.
Every customer that bought something at us is stored in customers. The parts field for a customer contains a list of parts the customer bought together on a certain date.
I would like to have an aggregate query in MongoDB that returns a mapping of which car parts were bought (bought_parts) from which customers. However, if the car_parts has the field sub_parts, the customer should show up for the subparts only.
So the query in the playground gives almost the correct result already, except for the sub_parts topic.
Example for customer_3:
{
"_id": "customer_3",
"parts": [
{
"bought_parts": [
3
],
date: "15.07.2020"
}
]
}
Since bought_parts has car_parts._id = 3:
{
"_id": 3,
"name": "steering wheel",
"sub_parts": [
1, // other car_parts._id s
2
]
}
The result should show customer_3 as a customer of car parts 1 and 2.
I'm not sure how to accomplish this, but I assume a "temporary" replacement of the id 3 in bought_parts with the actual ids [1,2] might solve it.
Expected output:
[
{
"_id": 1,
"customers": [
"customer_1",
"customer_2",
"customer_3" // <- since customer_3 bought car part 3 which has 1 in sub_parts
]
},
{
"_id": 2,
"customers": [
"customer_3" // <- since customer_3 bought car part 3 which has 2 in sub_parts
]
},
{
"_id": 3,
"customers": [
"customer_1", // <- since car_parts.id = 3 has [1, 2] in sub_parts, show customers of ids [1, 2]
"customer_2",
"customer_3"
]
},
{
"_id": 4,
"customers": [
"customer_1",
"customer_2"
]
}
]
Thanks a lot in advance!

Answer 1

EDIT: One way to do it is:
db.car_parts.aggregate([
{
$project: {
topLevel: {$concatArrays: [{$ifNull: ["$sub_parts", []]}, ["$_id"]]},
sub_parts: 1
}
},
{$unwind: "$topLevel"},
{
$group: {
_id: "$topLevel",
parts: {$push: "$_id"},
sub_parts: {$first: "$sub_parts"}
}
},
{
$project: {
parts: {$concatArrays: [{"$ifNull": ["$sub_parts", []]}, "$parts"]}
}
},
{
$lookup: {
from: "customers",
localField: "parts",
foreignField: "parts.scanned_parts",
as: "customers"
}
},
{$project: {customers: "$customers._id"}}
])
As you can see working on this playground.
Since you said there is only one level of sub-parts, I used another idea: creating a top level before the $lookup. Since you want customers that used part 3 for example, to be registered under parts 1,2 which are sub-parts of 3, the idea is to group them. This connection is a bit clumsy after the $lookup, but if we use the data that we have on the car_parts collection before the $lookup, we actually knows already that parts 1,2 are subpart of 3. Creating a topLevel temporary field, allows to group, in advance, all the parts and sub-parts that if a customer used on of them, he should be registered under this top level part. This makes things much more elegant...