how to use $subtract in $group in mongodb - mongodb

Question

I have below mongo record in purchase table with individual quantity
{
"_id" : ObjectId("60f0a8fa7f73e1b4883e2b26"),
"status" : "Sold",
"isSold" : true,
"unitcost" : 4.5,
"mrp" : 220,
"unitprice" : 6,
"product" : ObjectId("60f0a7c8355781b277607f19"),
"manufacturingdate" : ISODate("2021-06-30T18:30:00.000Z"),
"expirydate" : ISODate("2021-07-29T18:30:00.000Z"),
"customer" : ObjectId("60f0975a93ba0ba2152a1ec0")
},
{
"_id" : ObjectId("60f0a8fa7f73e1b4883e2b87"),
"status" : "Sold",
"isSold" : true,
"unitcost" : 4.5,
"mrp" : 220,
"unitprice" : 6,
"product" : ObjectId("60f0a7c8355781b277607f19"),
"manufacturingdate" : ISODate("2021-06-30T18:30:00.000Z"),
"expirydate" : ISODate("2021-07-29T18:30:00.000Z"),
"customer" : ObjectId("60f0975a93ba0ba2152a1ec0")
}
Want data like below Product and customer from there collection and group by product
{
"productDetails":{
"_id":ObjectId("60f0a7c8355781b277607f19"),
"title":"Prodcut 1"
},
"profit":3.0,
"unitprice":6,
"unitcost":4.5,
"customerDetails":{
"_id": ObjectId("60f0975a93ba0ba2152a1ec0"),
"title":"User 1"
}
}

Answer 1

pls refer to Sample code below and also find it here https://mongoplayground.net/p/S9xbt9Lnk5d :
db.purchase.aggregate([
{
"$addFields": {
"profit": {
"$subtract": [
"$unitprice",
"$unitcost"
]
}
}
},
{
"$group": {
"_id": "$product",
"profit": {
"$sum": "$profit"
},
"unitprice": {
"$first": "$unitprice"
},
"unitcost": {
"$first": "$unitcost"
},
"customerDetails": {
"$push": {
_id: "$_id"
}
},
}
}
])