MongoDB: Remove field in array with $lookup localField - mongodb

Question

I am beginner with MongoDB. I use $lookup in aggregation and use localField to get reference document.
db.orders.insert([
{ "_id" : 1, "item" : ['almonds','pecans','bread'], "price" : 12, "quantity" : 2 },
{ "_id" : 2, "item" : ['cashews','catty'], "price" : 20, "quantity" : 1 }
])
I tried to use $lookup and localField in aggregation but I can't find way to remove field _id and description
db.inventory.insert([
{ "_id" : 1, "sku" : "almonds", description: "product 1", "instock" : 120 },
{ "_id" : 2, "sku" : "bread", description: "product 2", "instock" : 80 },
{ "_id" : 3, "sku" : "cashews", description: "product 3", "instock" : 60 },
{ "_id" : 4, "sku" : "pecans", description: "product 4", "instock" : 70 },
{ "_id" : 5, "sku": "catty", description: "Incomplete", "instock" : 100 },
{ "_id" : 6 }
])
Expected results:
[
{
"_id" : 1,
"item" : [
{ "sku" : "almonds", "instock" : 120 },
{ "sku" : "pecans", "instock" : 70 },
{ "sku" : "bread", "instock" : 80 }
],
"price" : 12,
"quantity" : 2
},
{
"_id" : 2,
"item" : [
{ "sku" : "cashews", "instock" : 60 },
{ "sku" : "catty", "instock" : 100 }
],
"price" : 20,
"quantity" : 1
}
]

Answer 1

You can try lookup with aggregation pipeline,
$lookup join with inventory collection
$match to match is inventory sku in item array
$project to display required fields
db.orders.aggregate([
{
$lookup: {
from: "inventory",
as: "item",
let: { i: "$item" },
pipeline: [
{ $match: { $expr: { $in: ["$sku", "$$i"] } } },
{
$project: {
_id: 0,
sku: 1,
instock: 1
}
}
]
}
}
])
Playground