MongoDB aggregation on another aggreatation suggestions - mongodb

Question

I have a Json file imported into MongoDB. Every line on it is a user, and I have a field product, with the name of it. I know the value of every product, they are just few.
But this information is not stored on the Json.
I was able to do aggregation to retrieve the number of time that a user bought a product, but I would like to do a query to get directly the amount of money that each user spent.
This is my query:
db.source.aggregate([
{"$match": {
"$and":[
{"productName":{
"$in":[
"product2","product2","product3",
"product4","product5","product6"
]
}},
{ "$or": [
{"appID" : "nameOfAPP"},
{"appID": "NameOfAPP2"}
]}
]
}},
{ "$group": {
"_id": {
"id_user": "$id_user",
"productName": "$productName"
},
"count": { "$sum": 1}
}},
{ "$sort" : { "count": -1 } }
])
so the output is like that:
{ "_id" : { "id_user" : "user1", "productID" : "product2" }, "count" : 433 }
{ "_id" : { "id_user" : "user2", "productID" : "product1" }, "count" : 370 }
{ "_id" : { "id_user" : "user1", "productID" : "product3" }, "count" : 300 }
{ "_id" : { "id_user" : "user3", "productID" : "product6" }, "count" : 250 }
{ "_id" : { "id_user" : "user2", "productID" : "product5" }, "count" : 140 }
{ "_id" : { "id_user" : "user3", "productID" : "product4" }, "count" : 90 }
I know that product 1 costs 20$, product 2 costs 40$, product 3 costs 55$, product 4 costs -90$, product 5 costs 110$, product 6 costs 200$.
I would like to have an output like that:
{ "_id" : { "id_user" : "user1"}, "money_spent" : 600$ }
{ "_id" : { "id_user" : "user2"}, "money_spent" : 400$ }
etc
Can you help to get that result, I am new with MongoDB.
Thanks in advance.

Answer 1

If you cannot go to the original source data an are only working with an import then do this:
db.source.aggregate([
{"$match": {
"$and":[
{ "productName": {
"$in":[
"product1","product2","product3",
"product4","product5","product6"
]
}},
{ "$or": [
{"appID" : "nameOfAPP"},
{"appID": "NameOfAPP2"}
]}
]
}},
{ "$group": {
"_id": "$id_user",
"cost": {
"$sum": {
"$cond": [
{ "$eq": ["$_id.productId", "product1"] },
20,
{ "$cond": [
{ "$eq": ["$productName", "product2"] },
40,
{ "$cond": [
{ "$eq": [ "$productName", "product3"] },
55,
{ "$cond": [
{ "$eq": [ "$productName", "product4" ] },
-90,
{ "$cond": [
{ "$eq": [ "$productName", "product5" ] },
110,
200
]}
]}
]}
]}
}
}
}
}}
])
The $cond operator evaluates whether your field value matches the condition and places the appropriate value simply just $sum to get your result.
$cond provides a "ternary" operator or "if .. then .. else" that is used to evaluate the condition you provide in the first argument. You construct this to "cascade" where the condition evaluates to false in order to move on to the next condition to evaluate, otherwise return the value that matches your condition.
In this way your "known" values are applied as you aggregate for your expected total.