MongoDB aggregation project the specific fields from lookup - mongodb

Question

This example is following https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#use-lookup-with-mergeobjects
db.orders.insert([
{ "_id" : 1, "item" : "almonds", "price" : 12, "quantity" : 2 },
{ "_id" : 2, "item" : "pecans", "price" : 20, "quantity" : 1 }
])
db.items.insert([
{ "_id" : 1, "item" : "almonds", description: "almond clusters", "instock" : 120 },
{ "_id" : 2, "item" : "bread", description: "raisin and nut bread", "instock" : 80 },
{ "_id" : 3, "item" : "pecans", description: "candied pecans", "instock" : 60 }
])
Aggregation:
db.orders.aggregate([
{
$lookup: {
from: "items",
localField: "item", // field in the orders collection
foreignField: "item", // field in the items collection
as: "fromItems"
}
},
{
$replaceRoot: { newRoot: { $mergeObjects: [ { $arrayElemAt: [ "$fromItems", 0 ] }, "$$ROOT" ] } }
},
{ $project: { fromItems: 0 } }
])
Result:
{ "_id" : 1, "item" : "almonds", "description" : "almond clusters", "instock" : 120, "price" : 12, "quantity" : 2 }
{ "_id" : 2, "item" : "pecans", "description" : "candied pecans", "instock" : 60, "price" : 20, "quantity" : 1 }
Question: How to modify the aggregation to project the specific fields? e.g. project "_id", "item" and "description" only:
{ "_id" : 1, "item" : "almonds", "description" : "almond clusters" }
{ "_id" : 2, "item" : "pecans", "description" : "candied pecans" }

Answer 1

You're getting an empty array, because the $lookup catching anything.
match the types
$addFields to convert
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This should be the first stage:
{
$addFields: {
itemId: {
$convert: {
input: "$itemId",
to: "int"
}
}
}
},
If you prefer, there is no need to add a stage
You could also remove addFields and use $lookup+let.
Modify the lookup this way:
{
$lookup: {
from: "items",
let: {
itemId: {
$convert: {
input: "$itemId",
to: "int"
}
}
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$_id",
"$$itemId"
]
}
}
}
],
/** field in the items collection*/
as: "fromItems"
}
}
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