MongoDB - Copy field to another collection - mongodb

Question

Let's say I have collection orders:
[
{
"id":"1",
"items":{
"itemId":"1000",
"name":"Item 1",
"status":"STATUS"
}
},
{
"id":"2",
"items":{
"itemId":"1000",
"name":"Item 1",
"status":"ANOTHER_STATUS"
}
}
]
I have another collection item_projections which is
[
{
"id":"1",
"itemId":"1000",
"name":"Item 1",
"orderId":"1"
},
{
"id":"1",
"itemId":"1000",
"name":"Item 1",
"orderId":"2"
}
]
For every item from collection orders I would like to copy the field status to projection with matching on order id and itemId to have
[
{
"id":"1",
"itemId":"1000",
"name":"Item 1",
"orderId":"1",
"status":"STATUS"
},
{
"id":"1",
"itemId":"1000",
"name":"Item 1",
"orderId":"2",
"status":"ANOTHER_STATUS"
}
]
Is it possible to do it with aggregation lookup and merge pipelines?

Answer 1

Solution 1
$lookup - item_projections collection (key: orderId) join orders collection (key: id) and return orders array field with the document(s) of status field only.
$replaceRoot - Replace input document with new document.
2.1. $mergeObjects - Merge root document with the first document of orders.
$unset - Remove orders field.
db.item_projections.aggregate([
{
"$lookup": {
"from": "orders",
"localField": "orderId",
"foreignField": "id",
"pipeline": [
{
$project: {
status: "$items.status"
}
}
],
"as": "orders"
}
},
{
$replaceRoot: {
newRoot: {
$mergeObjects: [
"$$ROOT",
{
$first: "$orders"
}
]
}
}
},
{
$unset: "orders"
}
])
Sample Mongo Playground (Solution 1)
Solution 2
Or you can replace the 2nd and 3rd stages with $project.
{
$project: {
"id": 1,
"itemId": 1,
"name": 1,
"orderId": 1,
"status": {
$first: "$orders.status"
}
}
}
Sample Mongo Playground (Solution 2)