Aggregate Budgets by Customers - mongodb

Question

I have two collections: customers and budgets.
I need to get all customers with the related budgets inside an array.
My problem is, I need to start the aggregate from the budgets collection.
Also, I need to return customers who don't have any budgets related.
I need a list with something like this:
customer: {
Id: Guid,
Name: string,
CpfCnpj: number,
AccountantId: Guid
Budgets: []
}
How can I do that?
Here the example

Answer 1

You may want to "$project"/etc. the results somewhat differently, but here's one way to output a document for each customer with an array of their budgets (or an empty array if there is no budgets document for them).
db.budgets.aggregate([
{ // get customer docs with possible budgets
"$unionWith": {
"coll": "customers",
"pipeline": [
{
"$lookup": {
"from": "budgets",
"localField": "_id",
"foreignField": "CustomerId",
"as": "budgets"
}
}
]
}
},
{ // only keep budgets with a customer
"$match": {
"name": {"$exists": true}
}
},
{ // "budgets" set to empty array if missing
"$set": {
"budgets": {
"$ifNull": ["$budgets", [] ]
}
}
}
])
Try it on mongoplayground.net.
If "$unionWith" (introduced in MongoDB version 4.4) is unavailable, here's another way to do it by "transforming" (first four stages below) the original queried collection (here, budgets) into the desired collection (here, customers). The remainder of the pipeline is a simple "$lookup" to get the desired info.
db.budgets.aggregate([
{"$limit": 1},
{
"$lookup": {
"from": "customers",
"pipeline": [],
"as": "customers"
}
},
{"$unwind": "$customers"},
{"$replaceWith": "$customers"},
{
"$lookup": {
"from": "budgets",
"localField": "_id",
"foreignField": "CustomerId",
"as": "budgets"
}
}
])
Try it on mongoplayground.net.