Mongo - Aggregate Group Lookup Values - mongodb

Question

I'm trying to sum up values that are listed within a lookup. I have 2 collections:
Subscriptions (Pseudocode)
{
_id,
purchaseDate: Date,
wholesalePrice: Number,
purchasePrice: Number,
retailPrice: Number,
userID: String,
}
Payments
{
_id,
statementMonth: String, // e.g. 2020-08 (same as grouped id in Subscriptions)
paymentDate: Date,
userID: String,
amountPaid
}
Users will need to transfer profit margin value at end of month. Therefore, I want to create an output for Statements. These will have all Subscriptions and Payments grouped into monthly records, with a summary of all the data within them. I have managed to create everything up to the first group, however once I do my lookup to get Payment details, everything seems to fail.
This is my current pipeline
{
"$match": {
"userID": [provided UserID]
}
},
{
"$group": {
"_id": {
"$dateToString": {
"format": "%Y-%m",
"date": "$purchaseDate"
}
},
"totalWholesalePrice": {
"$sum": "$wholesalePrice"
},
"totalPurchasePrice": {
"$sum": "$purchasePrice"
},
"count": {
"$sum": 1.0
}
}
},
{
"$addFields": {
"totalAmountDue": {
"$subtract": [
"$totalPurchasePrice",
"$totalWholesalePrice"
]
}
}
},
{
"$lookup": {
"from": "transactions",
"localField": "_id",
"foreignField": "statementMonth",
"as": "transactions"
}
},
{
"$unwind": {
"path": "$transactions",
"preserveNullAndEmptyArrays": true
}
},
{
"$sort": {
"_id": -1.0
}
}
This returns 2 records if 2 transactions:
{
"_id" : "2020-08",
"totalWholesalePrice" : NumberInt(89),
"totalPurchasePrice" : 135.55,
"count" : 8.0,
"totalAmountDue" : 46.55,
"transactions" : {
"_id" : ObjectId("5f3faf2216d7a517bc51bfae"),
"date" : ISODate("2020-04-20T11:23:40.284+0000"),
"statementMonth" : "2020-08",
"merchant" : "M1268360",
"amountPaid" : "40"
}
}
{
"_id" : "2020-08",
"totalWholesalePrice" : NumberInt(89),
"totalPurchasePrice" : 135.55,
"count" : 8.0,
"totalAmountDue" : 46.55,
"transactions" : {
"_id" : ObjectId("5f3fc13f16d7a517bc51c047"),
"date" : ISODate("2020-04-20T11:23:40.284+0000"),
"statementMonth" : "2020-08",
"merchant" : "M1268360",
"amountPaid" : "2"
}
}
I would like final JSON to be:
{
"_id" : "2020-08",
"totalWholesalePrice" : NumberInt(89),
"totalPurchasePrice" : 135.55,
"count" : 8.0,
"totalAmountDue" : 46.55,
"transactions" : [{
"_id" : ObjectId("5f3faf2216d7a517bc51bfae"),
"date" : ISODate("2020-04-20T11:23:40.284+0000"),
"statementMonth" : "2020-08",
"merchant" : "M1268360",
"amountPaid" : "40"
}
{
"_id" : ObjectId("5f3fc13f16d7a517bc51c047"),
"date" : ISODate("2020-04-20T11:23:40.284+0000"),
"statementMonth" : "2020-08",
"merchant" : "M1268360",
"amountPaid" : "2"
}],
"totalPaid" : 42,
"totalBalance" : 4.55,
}

Answer 1

You need to add one more pipeline
db.collection.aggregate([
{
$group: {
"_id": "$id",
"transactions": { //Grouping transactions together
"$addToSet": "$transactions"
},
"totalWholesalePrice": {//As all values are unique getting the first
"$first": "$totalWholesalePrice"
}
//Similarly add the needed fields - use $first, $addToSet
}
}
])
playground
To get the total:
db.collection.aggregate([
{
$group: {
"_id": "$id",
"transactions": {
"$addToSet": "$transactions"
},
"totalWholesalePrice": {
"$first": "$totalWholesalePrice"
},
"totalAmountPaid": {
$sum: {
$toInt: "$transactions.amountPaid" //you stored as string, convert to appropriate type
}
}
}
}
])
playground