How can I group by a string instead of ObjectId in MongoDB aggregate? - mongodb

Question

I have two collections and a many-to-one relationship between them:
Product:
"_id" : ObjectId("61cc81c9585946c3b44f24411"),
"name" : "some random name",
"price" : 100,
"description" : "description",
"category_id" : ObjectId("61cc8100585946c3b44f2317d")
Category:
{
"_id" : ObjectId("61cc8100585946c3b44f2317d"),
"description" : "Category description",
"name" : "Electronics"
}
I would like to output the maximum product price for each category:
db.product.aggregate([
{ "$group": {
"_id": "$category_id",
"max": { "$max": "$price"}
}}
])
This works just fine as it prints me the following:
{ "_id" : ObjectId("61cc80fb585946c3b44f697c"), "max" : 62}
{ "_id" : ObjectId("61cc8100585946c3b44f697d"), "max" : 100}
But is there a way to get the "name" from the Category instead of its object id?
I know in SQL you would group by category_name but it does not seem to work here.

Answer 1

As suggested by #prasad, you should make use of $lookup stage after your $group stage.
db.product.aggregate([
{
"$group": {
"_id": "$category_id",
"max": {
"$max": "$price"
}
}
},
{
"$lookup": {
"from": "category",
"localField": "_id",
"foreignField": "_id",
"as": "categoryName",
}
},
{
"$set": {
"categoryName": {
"$arrayElemAt": [
"$categoryName.name",
0
]
}
}
}
])
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