Join MongoDB subdocuments in same parent document - node.js

Question

Given this Orders collection:
// Order documents
[
{
_id: "order_123",
items: [
{ _id: "item_123", type: "T-Shirt" },
{ _id: "item_234", type: "Hoodie" },
{ _id: "item_345", type: "Hat" },
],
refunds: [
{
_id: "refund_123",
items: ["item_123", "item_234"],
},
{
_id: "refund_234",
items: ["item_345"],
},
],
},
]
Is it possible to map refunds.items -> items._id, allowing us to filter by type?
This is how we currently get the refund sub-documents:
db.orders.aggregate([
{
$replaceRoot: {
newRoot: {
order: "$$ROOT",
refunds: "$$ROOT.refunds",
},
},
},
{
$unwind: "$refunds",
},
{
$project: {
order: "$order",
refund: "$refunds",
},
},
]);
Which gives us:
// Refund documents
[
{
refund: {
_id: "refund_123",
items: ["item_123", "item_234"],
},
order: { ... }, // The original order document
},
{
refund: {
_id: "refund_234",
items: ["item_345"],
},
order: { ... }, // The original order document
},
]
From here, we want to map up refund.items -> order.items._id to produce the following output:
[
{
_id: "refund_123",
items: [
{ _id: "item_123", type: "T-Shirt" },
{ _id: "item_234", type: "Hoodie" },
],
},
{
_id: "refund_234",
items: [
{ _id: "item_345", type: "Hat" }
],
},
]
Allowing us to filter refund documents by type.

Answer 1

You can do this using $unwind and $filter,
$unwind deconstruct array refunds
$project to show refund id in _id, and filter items that are in refunds.items array using $filter
db.orders.aggregate([
{ $unwind: "$refunds" },
{
$project: {
_id: "$refunds._id",
items: {
$filter: {
input: "$items",
cond: { $in: ["$$this._id", "$refunds.items"] }
}
}
}
}
])
Playground